(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.