High Kingship of Ireland was often claimed by the most powerful dynasties: the O’Neills (Ulster), O’Connors (Connacht), O’Briens (Munster), and MacMurroughs (Leinster).
Each clan was led by its own head of the family or Ceann Fine. The bigger clans had a Taoiseach or Chief and the smaller clans had Chieftains. Although clans were mostly blood related the size and power of each clan could change over time as other clans were absorbed into more powerful clans for economic as well as political reasons. Whatever the reason for joining all members took the same clan name.
The Gaelic order weakened after the Norman invasion (1169), and later English conquest eroded Gaelic power, but many of these clan names remain common surnames in Ireland today.
Ancient Irish Clans by Province
Ulster (North)
O’Neill (Ó Néill) – Kings of Tyrone; descended from Niall of the Nine Hostages.
O’Donnell (Ó Domhnaill) – Lords of Tyrconnell (Donegal); strong allies and rivals of the O’Neills.
Ó Rourke (Ó Ruairc) – Chiefs of Breifne (Leitrim).
Ó Reilly (Ó Raghallaigh) – Chiefs of East Breifne (Cavan).
MacGuinness (Mag Aonghusa) – Lords of Iveagh (Down).
MacMahon (Mac Mathghamhna) – Lords of Oriel (Monaghan).
Connacht (West)
O’Connor (Ó Conchobhair) – Kings of Connacht; provided several High Kings of Ireland.
Ó Kelly (Ó Ceallaigh) – Lords of Uí Maine (Galway/Roscommon).
Ó Malley (Ó Máille) – Sea-lords of Mayo; famous for Gráinne Mhaol (Grace O’Malley).
MacDermot (Mac Diarmada) – Lords of Moylurg (Roscommon).
Ó Flaherty (Ó Flaithbheartaigh) – Lords of Iar Connacht (west Galway).
Munster (South)
O’Brien (Ó Briain) – Descendants of Brian Boru, High King (d. 1014); rulers of Thomond (Clare, Limerick).
MacCarthy (Mac Cárthaigh) – Kings of Desmond (Cork/Kerry); rivals of the O’Briens.
O’Sullivan (Ó Súileabháin) – Powerful lords allied with the MacCarthys.
O’Donovan (Ó Donnabháin) – Lords in West Cork.
O’Keeffe, O’Callaghan, O’Mahony – Influential Munster clans.
Leinster (East)
MacMurrough-Kavanagh (Mac Murchadha Caomhánach) – Kings of Leinster; descendants of Diarmait Mac Murchadha (who invited the Normans).
O’Byrne (Ó Broin) – Lords of Wicklow.
O’Toole (Ó Tuathail) – Lords of Wicklow.
O’Dempsey (Ó Díomasaigh) – Lords of Clanmalier (Offaly/Leix).
O’Moore (Ó Mórdha) – Lords of Leix (Laois).