The top 100 Irish last names explained

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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.

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