Top 100 Irish last names explained
IrishCentral has put together a list of the top 100 common Irish surnames.
IrishCentral.com has put together a list of the top 100 common Irish surnames with a little explanation of where these names come from.
Whether you’re looking to trace your family crest or trying to trace your family roots this list will point you in the right direction.
From Aherne to Whelan here is our top 100 Irish names:
Aherne — (Ó hEachtighearna/Ó hEachthairn) (each, steed tightearna, lord). Originally Dalcassian, this sept migrated from east Clare to Co. Cork. In County Waterford the English name Hearn is a synonym of Hearn.
MacAleese — MacGiolla (son of the devotee of Jesus). The name of a prominent Derry sept. There are many variants of the name such as MacIliese, MacLeese, MacLice, MacLise, etc. The best known of this spelling, the painter Daniel MacLise, was a family of the Scottish highlands, know as MacLeish, which settled in Cork.
Allen — This is usually of Scottish or English origin but sometimes Ó hAillín in Offaly and Tipperary has been anglicized Allen as well as Hallion. Occasionally also in Co. Tipperary. Allen is found as a synonym of Hallinan. As Alleyn it occurs frequently in mediaeval Anglo Irish records. The English name Allen is derived from that of a Welsh saint.
MacAteer — Mac an tSaoir (saor, craftsman) An Ulster name for which the Scottish MacIntyre, of similar derivation, is widely substituted. Ballymacateer is a place-name in Co. Armagh, which is its homeland. Mac an tSaoir is sometimes anglicized Wright in Fermanagh.
MacAuley — Awley. There are two distinct septs of this name, viz. MacAmhalghaidh of Offaly and West Meath, and the more numerous MacAmhlaoibh, a branch of the MacGuires which as MacAmhlaoibh gives the form Gawley in Connacht. Both are derived from personal names. The latter must not be confused with MacAuliffe.
MacAuliffe — Mac Amhlaoibh. An important branch of the McCarthys whose chief was seated at Castle MacAuliffe. The name is almost peculiar to south-west Munster.
Barry — de Barra. The majority of these names are of Norman origin, i.e. de Barr (a place in Wales); they became completely hibernicized. Though still more numerous in Munster than elsewhere the name is widespread throughout Ireland. Barry is also the anglicized form of Ó Báire (see under Barr) and Ó Beargha (meaning spear-like according to Woulfe) a small sept of Co. Limerick.
Blake — deBláca (more correctly le Bláca). One of the ‘Tribes of Galway’ an epithet name meaning black which superseded the original Cadell. They are descended from Richard Caddell, Sheriff of Connacht in 1303. They became and long remained very extensive landowners in Co. Galway. Branch settled in Co. Kildare where their name is perpetuated in three town lands called Blakestown.
Brennan — Ó Braonáin. (The word braon has several meanings, possibly sorrow in this case). The name of four unrelated septs, located in Ossory, east Galway, Kerry and Westmeath. The county Fermanagh sept of Ó Branáin was also anglicized Brennan as well as Brannan.
O’Brien — Ó Briain. A Dalcassian sept, deriving its name from historical importance from the family of King Brian Boru. Now very numerous in other provinces as well as Munster, being the fifth most numerous name in Ireland. In some cases O’Brien has been made a synonym of O’Byrne and others of the Norman Bryan.
Browne — De Brún, more correctly le Brún (brown). One of the Tribes of Galway. Other important families of Browne were established in Ireland from the Anglo-Norman invasion onwards. The Browns of Killarney, who came in the sixteenth century, intermarried with the leading Irish families and were noted for their survival as extensive Catholic landowners throughout the period of the Penal Laws (The Kenmare associated with their name is in Co. Limerick) The Browne family shown on the map in Co. Limerick is of Camus and of earlier introduction. Yet another important family of the name was of the Neale, Co. Mayo. In that county Browne has also been used as a synonym of (O) Bruen.
Burk — de Burgh de Búrca. This one of the most important and most numerous Hiberno-Norman names. First identified with Connacht it is now numerous in all the provinces (least in Ulster). Many sub-septs of it were formed called MacHugo, MacGibbon, Mac Seoinín (Jennings), MacRedmond, etc.
Butler — Always called deBuitléir in Irish, though it is of course properly le Butler not de. It is one of the great Norman-Anglo which, however, did not soon become hibernicized like the Burkes, etc. Historically it is mainly identified with the Ormond country. It is now very numerous in all the provinces except Ulster.
MacCabe — Mac Cába. A galloglass family with the O’Reillys and the O’Rourkes which became a recognized Breffny sept. Woulfe suggests cába, cape, a surname of the nickname. Having regards to their origin it is more likely to be from a non-Gaelic personal name.
Callaghan — Ó Ceallacháin The derivation from ceallach, strife, which usually given, is questioned but no acceptable alternative has been suggested. The eponymous ancestor in this case was Ceallacháin, King of Munster (d. 952). The sept was important in the present Co. Cork until the seventeenth century and the name is still very numerous there. The chief family was transplanted under the Cromwellian regime to east Clare, where the village of O’Callghan’s Mills is called after them.
Campbell — Mac Cathmhaoil (cathmhaoil, battle chief). An Irish sept in Tyrone; in Donegal it is usually of Scottish galloglass origin, viz. Mac Ailín a branch of the clan Campbell (whose name is from cam béal, crooked mouth) Many Campbells are of more recent Scottish immigrants. See MacCawell. The name has been abbreviated to Camp and even Kemp in Co. Cavan.
MacCarthy — Mac Ćarthaigh (cárthach, loving). The chief family of the Eoghanacht and one of the leading septs of Munster, prominent in the history of Ireland from the earliest times to the present. MacCarthy is the most numerous Mac name in Ireland.
Cassidy — Ó Caiside A Fermanagh family of ollavs and physicians to the Maguires. Now numerous in all the provinces except Connacht.
Clery — Cleary Ó Cléirigh (cléireach, clerk) One of the earliest hereditary surnames. Originally of Kilmacduagh (Co. Galway) the sept was dispersed and after the thirteenth century settled in several parts of the country; the most important branch were in Donegal where they became notable as poets and antiquaries. In modern times the name is found mainly in Munster and Dublin.
O’Connor — ÓConchobhair. The name of six distinct and important septs. In Connacht there were O’Connor and O’Conor Don (of which was the last High King of Ireland) with its branches O’Conor Roe and O’Conor Sligo; Also O’Conor Faly (i.e. of Offaly), O’Connor Kerry and O’Connor of Corcomroe (north Clare). The prefix, O, formerly widely discarded, has been generally resumed. Similarly the variant from Connors has been O’Connor again.
(O) Conroy — Conree, Conary, Conry. These mainly Connacht names, owing to the similarity to of the anglicized forms, have become virtually indistinguishable. They represent four Gaelic originals, viz. Mac Conraoi (Galway and Clare), Ó Conraoi (Galway), Ó Conaire (Munster and Ó Maolchonaire (an important literary family of Co. Roscommon)
Cooney — Ó Cuana (for the probable derivation see Coonan). Originally of Tyrone this family later migrated to north Connacht. The Cooneys of east Clare and south-east Galway may be of different origin.
MacCormack — Cormick Mac Cormaic. This like MacCormican is formed from the forename Cormac. This name is numerous throughout all the provinces, the spelling MacCormick being more usual in Ulster. For the most part it originated as a simple patronymic; the only recognized sept of the name was of the Fermanagh-Longford area. Many of the MacCormac(k) families of Ulster are of Scottish origin, being a branch of the clan Buchanan-MacCormick of MacLaine.
Daly — Dawley Ó Dálaigh (dálach, from dáil, assembly). One of the greatest names in Irish literature. Originally West Meath, but sub-septs in several different localities as Map. As that in Desmond appears in the records as early as 1165 it is probable that this was a distinct sept.
Darcy — Ó Dorchaidhe (dacha, dark) One of the ‘Tribes of Galway’ also anglicized Dorsey, it is the name of two septs, one in Mayo and Galway, the other in Co. Wexford.
(O) Delaney — Ó Dubhshláine (another case of dubh, black-Sláine, perhaps the river Slaney) The prefix O has been completely discarded in the anglicized form of the name. It appears as Delane in Mayo. Both now and in the past it is of Leix and Kilkenny.
(O) Dempsey — Ó Díomasaigh (díomasach, proud). A powerful sept in Clanmalier. O’Dempsey was one of the very few chiefs who defeated Strongbow in a military engagement. Many of his successors distinguished themselves as Irish patriots and they were ruined as a result of their loyalty to James II. The name is now numerous in all the provinces.
Disney — Derived from a French place-name and originally written D’Isigny etc., the name Disney occurs quite frequently in the records of several Irish counties in the south and midlands since the first half of the seventeenth century.
(O) Dolan — The general accepted form in Irish today is Ó Dúbhláin (mod. Ó Dúláin) as given by Woulfe and others. O’Dolean, later Dolan, derives from Ó Dobhailen the name of a family on record since the twelfth century in the baronies of Clonmacnowen, Co. Galway, and Athlone, Co. Roscommon, in the heart of the Uí Mainecountry and quite distinct from Ó Doibhilin (Devlin). There has been a movement north-eastwards so that now the name Dolan is numerous in Co. Leitrim, Fermanagh, and Cavan as well Co. Galway and Roscommon.
Mac Donagh — Mac Donnchadha (son of Donagh). A branch of the MacDermots of Connacht where the name is very numerous. In Connemara the name is usually that of a branch of the O’Flahertys. The MacDonagh sept in Co. Cork were a branch of the McCarthys: the name is now rare there and apparently many of these resumed the name MacCarthy.
O’Donnell — Ó Domhnaill. The main sept, one of the most famous in Irish history, especially in the seventeenth century, is of Tirconnell; another is of Thomond and a third of the Uí Maine.
(O) Donoghue — Donohoe ÓDonnchadha. An important sept in Desmond: where the name was perpetuated in the territory called Onaght O’Donoghue. There also were two others in County Galway and Co. Cavan where the spelling Donohoe is usual. According to Dr. John Ryan there was another O’Donoghue sept in Co. Tipperary of Eoghanacht. Descent.
Mac Dowell — Mac Dubhghaill (dubh, black-gall, foreigner) This is the Irish from of the name of the Scottish family of Macdugall which came from the Hebrides of galloglasses, and settled in Co. Roscommon where Lismacdowell locates them. It is now mainly found in north Ulster, largely due to more recent immigration.
(O) Duffy — ÓDubhthaigh. A numerous name in all the provinces except Munster. Modern statistics show that is now the most numerous name in Co. Monaghan.
(O) Dwyer — Ó Duibhir (dubh and odhar, gen. uidhir, duncoloured). Of Kilnamanagha, a leading sept in mid-Tipperary. A great name is resistance to English domination.
Mac Fadden — Fayden Mac Pháidín(Paídí n, a diminutive of Pádraig, Patrick). An Ulster name, of both Scottish and Irish origin. Without the Mac it is found in Mayo.
Fanning — Fannin Fainín. A Name of Norman origin prominent in Co. Limerick where Fanningstown, formerly of Ballyfanning, indicates the location. They were formerly of Ballingarry, Co. Tipperary, where in the fifteenth century the head of the family was, like Irish chiefs, officially described as ‘captain of his nation’. Fannin is a variant.
Fitzgerald — Mac Gerailt. One of the two greatest families which came to Ireland as a result of the Anglo-Norman invasion. It had two main divisions, Desmond (of whom are the holders of the ancient titles Knight of Kerry and Knight of Glin); and Kildare, whose leaders held almost regal sway up to the time of the Rebellion of Silken Thomas and the execution of Henry VIII of Thomas and his near relatives in 1537. The bane is now very numerous.
Fitzpatrick — Mac Giolla Phádraig (devotee of St. Patrick). The only Fitz name of Gaelic-Irish origin, the main sept being located in Ossory. The name is numerous also in Fermanagh where families so called are said to be of MacGuire stock.
Flanagan — Ó Flannagáin (flann, ruddy or red). Of the several septs of the name that of Connacht is the most important: their chief ranked as one of the ‘royal lords’ under O’Connor, King of Connacht.
Flood — Some Floods are of English extraction, but in Ireland they are plainly Ó Maoltuile or Mac Maoltuile, abbreviated to Mac an Tuile and Mac Tuile anglicized MacAtilla or MacTully as well as Flood. Tuile means flood but probably it is here for toile, gen. of toil, will, i.e. the will of God. In parts of Ulster Flood is used for the Welsh Floyd. (Welsh llwyd. Grey)
(O) Flynn — Flyng ÓFloinn (flann, ruddy). This numerous and widespread name originated in a number of different places, including Kerry and Clare. Of the two in Co. Cork one was a branch of the Corca Laoidhe, the other, lords of Muskerylinn (Muiscre Uí Fhloinn); in north Connacht the O’Flynns were leading men under the royal O’Connors, and there was also an erenagh family there; while further West on the shores of Lough Conn another distinct erenagh family was located. For the name in Ulster is an indigenous sept.
(O) Gallagher — ÓGallchobhair. This name (gallchobhar, foreign help) has at least 23 variant spellings in anglicized forms, several of them beginning with Gol instead of Gal. It is that of one of the principal septs of Donegal.
MacGowan — Mac an Ghabhann, Mac Gabhann. In Co. Cavan, the homeland of this sept, the name has been widely changed by translation to Smith (though Smithson was a truer translation); but in outlying areas of Breffny MacGowan is retained.
(O) Grady — Ó Grádaigh (gráda, illustrious). A Dalcassian sept. The leading family went to Co. Limerick but the majority are still Clare where the prefix O is retained more than anywhere else. An important branch changed their name to Brady in the late sixteenth century. The well-known name Grady has to a large extent absorbed the rarer Gready which is properly a Mayo name. This resulted in the name of Grady being numerous in north Connacht and adjacent areas of Ulster.
MacGrath — Magrath Mac Graith, Mag Raith. The personal name in this case is Craith not Raith. The name of two distinct septs; viz. (i) that of Thomond who supplied hereditary ollamhs in poetry to the O’Briens, a branch of whom migrated to Co. Wexford; and (ii) of Termon MacGrath in north-west Ulster, a co-arb family. MacGrath is often called MacGraw in Co. Down and MacGragh in Donegal.
(O) Hagan — Ó hÁgáin. It is fairly well established that this name was originally Ó hÓgáin (from óg, young). It is that of an important Ulster sept: the leading family was of Tullahogue. Ó hAodhagáin, also anglicized O’Hagan, is said to be a distinct sep of Oriel, but owing to proximity of Co. Tyrone and Armagh, they are now indistinguishable. The Offaly name mentioned by Woulfe is now extinct or absorbed by Egan in Leinster. ÉO Dowd (a) Ó Dubhda. A branch settled in Kerry where they are called Doody. Another small sept of Ó Dubhda Co. Derry and they are usually Duddy now.
Hanlon — Ó hAluain (possibly from luan, champion, intensified by an) One of the most important of the septs of Ulster. The present association of the name with West Munster is of comparatively recent inception.
O’Hara — Ó hEaghra. An important dual sept located in Co. Sligo, the chiefs being O’Hara Boy (buidhe) and O’Hara Reagh (riabhach). A branch migrated to the glens of Antrim.
(O) Healy — Hely This is Ó hÉalaighthe in Munster, sometimes anglicized Healihy, and ÓhÉilidhe in north Connacht, derived respectfully from words meaning ingenious and claimant. Ballyhelyon Lough Arrow was the seat of the altar. The Munster sept was located in Donoughmore, Co. Cork, whence was taken the title conferred on the Protestant branch.
(O) Heaney — Heeney The Principal sept of this name is Ó hÉighnigh in Irish, important and widespread in Oriel, formerly stretching its influence into Fermanagh. Hegney is a variant. Another family of the name Ulster were erenaghs of Banagher in Co. Derry. Minor septs of Ó hÉanna (Éanna, old form of Enda), also anglicized Heaney, were of some note in Clare, Limerick, and Mayo up to the seventeenth century.
(O) Higgins — Ó hUigín (from an Old-Irish word akin to Viking, not from uige). A sept of the southern Uí Néill which migrated to Connacht. The O’Higgins father and son of South American fame came from Ballinary, Co. Sligo, not Ballina.
(O) Hogan — Ó hÓgain (og, young) Three septs are so called: one is Dalcassian and one of Lower Dormond (sometimes regarded as the same); there is also one of the Corca Laoidhe.
Kane — O Cahan Ó Catháin. As lords of Keenaght the O’Kanes were a leading sept in Ulster up to the time of the plantation of Ulster. The name is still very numerous in its original homeland.
Keating — One of the earliest Hibernicized Anglo-Norman families whose name was gaelicized Ćeitinn. They settled in south Leinster. The historian Dr. Geoffrey Keating was of C. Tipperary. The name with the prefix Mac is associated exclusively with the Downpatrick area, where MacKetian is a synonym of it. The theory that Keating is derived from Mac Eitienne is improbable. Woulfe makes it toponymic. The most acceptable suggestion is that it is from Cethyn, a Welsh personal name.
(O) Kelly — Ó Ceallaigh (The derivation of Kelly is uncertain: the most probable suggestion is that is from ceallach, strife) The most important and numerous sept of this name is that of the Uí Maine. Kelly is the second most numerous name in Ireland. In 1890 less than one percent of them had the prefix O but this has been to some extent resumed.
Mac Kenna — Kennagh Mac Cionaoith. A branch of the southern Uí Neill, mainly located in Co. Monaghan where they were lords of Truagh; the name is now fairly numerous also in Leinster and Munster. Locally in Clare and Kelly the last syllable is stressed, giving the variants Kennaw, Ginna, Gna, etc.
Kennedy— Ó Cinnéide (ceann, head-éidigh, ugly). An important Dalcassian sept of east Clare which settled in north Tipperary and spread thence far as Wexford whence came the family of President J.F. Kennedy. The Scottish Kennedys are by remote origin Irish Gaels.
Lawless — Laighléis (from the Old-English laghles, outlaw). The name, introduced into Ireland after the Anglo-Norman invasion, is now numerous in Co.s Dublin and Galway. It was one of the ‘Tribes of Kilkenny’ but has now no close association with the city.
(O) Leahy — Ó Laochdha (laochda, heroic). This name is very numerous in Munster but not elsewhere It is basically distinct from Lahy though they have been used synonymously.
(O) Leary — Ó Laoghaire. (Laoghaire was one of the best-known personal names of Ancient Ireland). A sept of the Corca Laoidhe established in Muskerry, of importance in all fields of national activity, especially in literature, and in the military sphere both at home and as the Wild Geese.
(O) Lemon — Lenna Ó Leannáin (possibly from leann, a cloak or mantle; leanán, paramour, has also been suggested). This is the name of several distinct septs located respectively in Co.s Cork, Fermanagh, and Galway. The last named is of the Sodhan pre-Gaelic stock. The Fermanagh family were erenaghs of Lisgoole. Ó Leannáin is also used as a synonym of Lineen (Ó Luinín), another Fermanagh erenagh family. Further confusion arises from the fact that these have been widely changed to the English name Leonard.
Mac Loughlin — Mac Lochlainn (from a Norse personal name). Of Inishowen. A senior branch of the Northern Uí Néill. They lost their early importance as a leading sept of Tirconnell in the thirteenth century, but are still very numerous in their original homeland-Co.s Donegal and Derry-where their name is usually spelt MacLaughlin; MacLoughlin, also numerous, is more widespread. Minor septs in Connacht were akin to the MacDermota and the O’Connors.
Mac Mahon — Mac Mathghamhna mod. Mac Mathúna (mathghamhan, bear) Thename of two septs, both of importance. That of Thomond descends from Mahon O’Brien, grandson of Brian Ború. MacMahon is now the most numerous name in Co. Clare. In later times the majority of the many of the name were from the Co. Monaghan, where McMahons are numerous today, though less so in Thomond.
(O) Malley — Mailey Ó Máille. (meall, peasant). A branch of the Cenél Eoghain located in Tyrone where their territory was known as ‘O’Mellan’s Country’. They were hereditary keepers of the Bell of St. Patrick.
O’Meara — Mara Ó Meadhra (meadhar, merry). This well-known sept, which has produced many distinguished men and women, gave its name to the village of Toomevara, which locates their homeland. This one of the few O names from which the prefix was never very widely dropped.
Molloy — Mulloy Ó Maolmmhuaidh. (The adjective muadh) denotes bit and soft as well noble). An important sept of Fercal in mid-Leister. Molly is an anglicized form of Ó Maolaoidh. Apart from five variant spellings, such as Maloy and Mulloy, Molloy has been officially recorded as synonym of Mulvogue (Connacht), Logue (Co. Donegal), Mullock (Offaly), Mulvihill (Kerry), and Slowey (Co. Monaghan) while Maloy has been used for MacCloy in Co. Derry.
(O) Moran — Apart from MacMorran of Fermanagh, which has inevitably been changed to Moran, there are a number of distinct septs of Ó Moráin and Ó Moghrain whose name is anglicized Moran. Four of these are of Connacht-in which province the name is much more numerous than elsewhere-originally located (a) at Elphin (akin to the O’Connors), (b) in Co. Leitrim (of the Muinitir Eolais), (c) in. Co. Mayo at Ardanee, (d) in Co. Galway, a minor branch of the Uí Maine. The Leitrim families are also called Morahan, as is the fifth to be enumerated, viz. that of Offaly, where Morrin is a synonym.
Moynihan — Ó Muimhneacháin (Muimhneach, Munsterman). Although there was a small sept of this name, sometimes changed to Munster, in Mayo, families so called belong almost exclusively to south-west Munster, Moynihan being very numerous on the borders of two counties. Minihan, another form of the name, is mainly found in Cork.
(O) Mulligan — Ó Maolagáin (probably a diminutive of maol, see MacMullen). An important sept in Donegal, much reduced at the time of the Plantation of Ulster and now found more in Co. Mayo and Monaghan.
(O) Murphy — Ó Murchadh. Murphy is the most numerous name in Ireland. The resumption of the prefixes O and Mac, which is a modern tendency with most Gaelic-Irish names, has not taken place in the case of Murphy.
(Mac) Nally — Mac Anally Mac an Fhailghih (failgheach, poor man). Without the prefix Mac this name now is found mainly in Mayo and Roscommon; with the Mac it belongs to Oriel. Woulfe says that the Mayo Nallys are of Norman or Welsh oigin and acquired a Gaelic name. This is unlikely in the case of the MacNallys of Ulster as there they are often called Mac Con Ulaidh (son of the hound of Ulidia, i.e. eastern Ulster). In the ‘census ‘ of 1659 it appears as MacAnully, MacEnolly, MacNally, and Knally, all in Oriel or in counties adjacent thereto.
Mac Namara — Mac Conmara (hound of the sea). The most important sept of the Dál gCais after the O’Briens to whom they were marshals.
(O) Nolan — Knowlan Ó Nualláin (nuall, shout) In early times holding hereditary office under the Kings of Leinster, the chief of this sept was known as Prince of the Foherta, i.e. the Barony of Forth, in the present county of Carlow where the name was and still is numerous. A branch migrated to east Connacht and Co. Longford, in Roscommon and Mayo Nolan is used synonymously with Holohan (from the genitive plural); and in Fermanagh as an Anglicized form of ÓhUltacháin (Hultaghan). There was also a sept of the name of Corca Laoidhe which is now well represented in Co. Kerry.
Prendergast — de Priondragás. One of the powerful families which came to Ireland at the time of the Anglo-Norman invasion. They are still found mainly in the places of their original settlement. Some of those in Mayo assumed the name FitzMaurice.
MacQuaid — Quade Mac Uaid (son of Wat). A well-known name in Co. Monaghan and adjacent areas. Without the prefix Mac the name is found in Co. Limerick.
(O) Rafferty — Ó Raithbheartaigh, mod. Ó Raifeartaigh. Though etymologically this (from rath bheartach, prosperity wielder) is distinct from Ó Robhartaigh (from robharta, full tide) anglicized O’Roarty, these two names have been treated as one, at least since the fifteenth century. As co-arbs of St. Columcille on Tory Island Roarty is now mainly Co. Donegal while Rafferty is of Co. Tyrone and Co. Lough.
(O) Rahilly — Ó Raithile this well-known Munster family originated as a branch of the Cenél Eoghain in Ulster buthas long been closely associated with west Munster, the poet Egan O’Rahilly for example was a Kerryman.
Redmond — Réamonn. A Hiberno-Norman family of importance throughout Irish history. They are associated almost entirely with South Wexford. The branch of the MacMurroughs in north of that county, some of whom adopted the name of Redmond whose chief was called Mac Davymore, are quite distinct from the MacRedmonds.
(O) Regan — Ó Riagain. Ó Réagainis used in county Waterford. There are three septs with this name. That shown as of Leix was in the early times one of the ‘Tribes of Tara’. The eponymous ancestors of the Thomond sept were akin to Brian Boru. The third was akin to the MacCarthys.
(O) Reilly — Ó Raghailligh One of the most numerous names in Ireland, especially so in Co. Cavan. The prefix O has been widely resumed in the anglicized form. The head of this important sept was chief Breffny O’Reilly
(O) Riordan — Rearden Ó Riordáin. This numerous sept belongs exclusively to Minster, he earlier form of Ó Rioghbhardáin reveals its derivation from riogh bhard, royal bard.
(O) Rooney — Ó Ruanaidh. Originating in Co. Down, where Ballyroney locates them, this name is now numerous in all the provinces except Munster. In West Ulster and north Connacht Rooney is often an abbreviation of Mulrooney.
(O) Shea — Shee Ó Séaghdha; mod. Ó Sé (séaghdha, hawklike, secondary meaning stately). Primarily a Kerry sept, but (as in Shee) it is notable as the only Gaelic-Irish name among “the Tribes of Kilkenny’ to which county and Co. Tipperary a branch of the sept migrated in the thirteenth century.
(O) Sheehan — Sheahan Ó Síodhacháin. (The obvious derivation from síodhach, peaceful, is not accepted by some Celtic scholars). The Dalcassian sept which spread southwards accounts for the majority of Sheehans who are now very numerous in Co.s Cork, Kerry, and Limerick. Formerly also there was an Uí Maine sept of this name which, however, is rarely found in Connacht today.
(O) Slattery — Ó Slatara, Ó Slatraigh (slatra, strong). Of Ballyslatterly in east Clare. The name has now spread to adjacent counties of Munster.
Smith — Smyth When not the name of an English settler family, Smith is usually a synonym of MacGowan, nearly always so in Co. Cavan.
(Mac) Spillan(e) — Mac Spealáin (derivation as O’Spillane). The family is, however, quite distinct from Ó Spealáin (O’Spillane) Spollan and Spollin, rarely retaining the prefix Mac, are numerous in County Offaly. Older anglicized forms were Spalane and Spalon.
(O) Sullivan — Ó Súileabhain. (While there is no doubt that the basic word is súil (eye) there is a disagreement as to the meaning of the last part of the name). This is the most numerous surname in Munster and is third in all of Ireland. Originally of south Tipperary, the O’Sullivans were forced westwards by the Anglo-Norman invasion where they became one of the leading septs of the Munster Eoghanacht. There were several sub-septs, of which O’Sullican Mor and O’Sullivan Baere were the most important.
(Mac) Sweeney — Swiney Mac Suibhne (the word suibhne denotes peasant, the opposite of diubhneI). Of all galloglass origin it was not until the fourteenth century that the three great Tirconnell septs of MaSweeney were established; more than a century later a branch went to Munster.
(O) Tierney — Ó Tighearnaigh (tighearna, lord). There were three septs of this name, in Donegal, Mayo, and Westmeath, but it is now scattered. It is much confused with Tiernan in Mayo. In southern Ulster this is usually of different origin, viz., Mac Giolla Tighearnaigh, which was formerly also anglicized MacIltierney.
Walsh — Breat(h)nach (Welshman) which is re-anglicized also as Brannagh, Brannick etc. A name given independently to many unconnected families in different parts of the country and now the fourth most numerous of all Irish surnames. It is sometimes spelt Welsh, which is the pronunciation of Walsh in Munster and Connacht.
(O) Whelan Ó Faoláin (faol, wolf). A variant form of Phelan numerous in the country between Co. Tipperary and Co. Wexford. Whelan is also sometimes an abbreviation of Whelehan and occasionally a synonym of Hyland. Whelan is rare in Ulster.
from: Irishcentral