Irish surnames starting with O

The O names in brief

• O'Brien · Ó Briain · Gaelic dynasty
  Descendant of Brian (Boru). The royal house of Thomond, descended from Brian Boru himself. Uniquely, they rode out every conquest — kings, then earls, never dispossessed.
• O'Callaghan · Ó Ceallacháin · Gaelic sept
  Descendant of Ceallachán of Cashel. Descended from a 10th-century king of Munster; transplanted by Cromwell from their Blackwater lands, yet the clan's chief line was still being formally recognised in the 20th century — in Spain.
• O'Connell · Ó Conaill · Gaelic sept
  Descendant of Conall — a name built from 'hound' and 'valour'. The great Kerry sept of Iveragh, seated at Derrynane on the edge of the Atlantic. Daniel O'Connell, 'the Liberator', built the first mass political movement in Europe out of penny subscriptions and won Catholic Emancipation in 1829 without firing a shot.
• O'Connor · Ó Conchobhair · Gaelic dynasty
  Descendant of Conchobhar, 'lover of hounds'. Six distinct royal septs — the greatest of Connacht gave Ireland its last two High Kings. The O'Conor Don line continues unbroken today at Clonalis House, keeping 100,000 ancestral documents.
• O'Dea · Ó Deághaidh · Gaelic sept
  Descendant of Deághaidh, a tenth-century chieftain. A Dalcassian sept of Clare seated at Dysert. At the Battle of Dysert O'Dea in 1318 Conor O'Dea and his allies destroyed the Norman de Clare army and ended English power in Thomond for two centuries.
• O'Donnell · Ó Domhnaill · Gaelic dynasty
  Descendant of Domhnall, 'world-ruler'. Kings of Tír Chonaill. Red Hugh escaped Dublin Castle through a snowstorm at 19 and set Ulster alight; his brother Rory's departure in 1607 was the Flight of the Earls.
• O'Donoghue · Ó Donnchadha · Gaelic dynasty
  Descendant of Donnchadh. Eóganacht royalty pushed from Cork into Kerry, where the O'Donoghue Mór ruled the lakes of Killarney — and, legend says, still rides across Lough Leane at dawn each May Day. The O'Donoghue of the Glens is a recognised chief to this day.
• O'Dowd · Ó Dubhda · Gaelic dynasty
  Descendant of Dubhda, 'the dark'. Kings of north Connacht whose inauguration required a white wand cut from a sacred tree; the Book of Lecan, treasure of Irish learning, was compiled under O'Dowd patronage.
• O'Driscoll · Ó hEidirsceoil · Gaelic sept
  Descendant of the eidirsceol — 'the go-between, the interpreter'. Sea-lords of the Corcu Loígde at Baltimore and Sherkin, taxing every fishing fleet that entered their waters. Their name means 'interpreter' — fitting for a family that dealt with Spain as easily as with Dublin.
• O'Farrell · Ó Fearghail · Gaelic sept
  Descendant of Fearghal, 'man of valour'. Princes of Annaly whose fortress gave Longford ('an longfort', the stronghold) its name. Their tomb-slabs still pave Abbeyshrule and Ardagh.
• O'Flaherty · Ó Flaithbheartaigh · Gaelic dynasty
  Descendant of Flaithbheartach, 'bright ruler'. Kings of Iar Connacht, Connemara and the lake country, after the Normans took the plains. Galway's west gate carried the prayer 'From the ferocious O'Flahertys, good Lord deliver us'; Gráinne O'Malley married their chief.
• O'Hagan · Ó hAodhagáin · Gaelic sept
  Descendant of Aodhagán, a diminutive of Aodh, 'fire'. Lords of Tullyhogue in Tyrone, where they held the hereditary right to inaugurate the O'Neill as king of Ulster — the man who made kings, rather than the king.
• O'Halloran · Ó hAllmhuráin · Gaelic sept
  Descendant of Allmhurán, 'the stranger from over the sea'. Two septs, by the Fergus in Clare and near Clarinbridge in Galway. Limerick's Sylvester O'Halloran, the great 18th-century surgeon-historian, was a founder of the school of surgery that became the RCSI.
• O'Hara · Ó hEaghra · Gaelic sept
  Descendant of Eaghra of Luighne. Lords of the Ox Mountains country; their poem-book, the Book of O'Hara, preserves the praise-poetry of a Sligo court. Scarlett O'Hara made the name world-famous from a fictional Tara.
• O'Hare · Ó hÍr / Ó hAichir · Gaelic sept
  Descendant of Ír, 'long-lasting' — or of Aichear, 'fierce'. An Oriel sept, kin to the O'Hanlons, whose name appears in the Annals of the Four Masters. Strongest still in south Armagh and north Louth.
• O'Keeffe · Ó Caoimh · Gaelic sept
  Descendant of Caomh, 'gentle, beloved'. Eóganacht royalty pushed west into Duhallow, where 'Pobal O'Keeffe' kept its own Gaelic customs so long it was called the last unconquered pocket of old Munster.
• O'Leary · Ó Laoghaire · Gaelic sept
  Descendant of Laoghaire, 'calf-keeper'. A Corcu Loígde sept settled in Uíbh Laoghaire, Inchigeela and the Lee valley. When Art Ó Laoghaire was shot in 1773 for refusing to sell his horse to a Protestant for £5, his widow Eibhlín Dubh composed the Caoineadh — often called the greatest poem in Irish.
• O'Mahony · Ó Mathghamhna · Gaelic dynasty
  Descendant of Mathghamhain, 'bear'. Descended from Cian, son-in-law of Brian Boru, the O'Mahonys held the wild peninsulas of west Cork with a dozen castles facing the Atlantic. Their annual gatherings still draw the clan home to Dunmanus and Mizen.
• O'Malley · Ó Máille · Gaelic sept
  Descendant of Máille — 'noble chief'. Terra marique potens, 'powerful by land and sea', runs their motto, and Gráinne Mhaol proved it: pirate queen of Clew Bay, who met Elizabeth I as one sovereign woman to another.
• O'Neill · Ó Néill · Gaelic dynasty
  Descendant of Niall (Glúndub). The premier dynasty of Gaelic Ireland, kings of Ulster for a thousand years. Hugh O'Neill's Nine Years' War came within one battle of reversing the conquest.
• O'Reilly · Ó Raghallaigh · Gaelic sept
  Descendant of Raghallach. Lords of East Bréifne, so commercially sharp they minted their own coinage, 'O'Reilly's money', and left their name in the phrase 'the life of Reilly'.
• O'Rourke · Ó Ruairc · Gaelic dynasty
  Descendant of Ruarc (a Norse name). Kings of Bréifne. Brian O'Rourke sheltered shipwrecked Armada men in 1588, was hanged at Tyburn for it, and requested, in vain, to be hanged with a gad, in the Irish manner, not a rope.
• O'Shea · Ó Séaghdha · Gaelic sept
  Descendant of Séaghdha, 'hawk-like, stately'. Chiefs of Iveragh on the Ring of Kerry; a merchant branch became the only Gaelic family among Kilkenny's 'Ten Tribes' — rich enough to build almshouses that still stand.
• O'Sullivan · Ó Súilleabháin · Gaelic sept
  Descendant of Súildubhán, 'dark-eyed one'. Princes of the Beara peninsula. After Kinsale, Donal Cam O'Sullivan Beare led a thousand followers on a terrible 300-mile winter march to Leitrim — 35 arrived.
• O'Toole · Ó Tuathail · Gaelic sept
  Descendant of Tuathal, 'ruler of the people'. Wicklow mountain lords beside the O'Byrnes; their saint, Laurence O'Toole, was Dublin's archbishop during the Norman invasion and died its heartbroken peacemaker.

Also spelled with O

These are listed on the atlas under another spelling, but the O form is the one many families carry.