Irish Families and their alternate spellings. Data from Edward MacLysaght's Irish Families book.
O'Aherne (Hearne, O'Heffron)
Athy,
MacAuliffe
MacAwley (Cauley, Magawley)
Barrett (MacPadine, MacEvilly, Staunton)
Barry
O'Beirne
Blake (Caddell, Blowick)
Bodkin
O'Boland (Bolan)
O'Boylan (Boyland)
O'Boyle
MacBrady
O'Brallaghan (Bradley)
O'Breen (MacBreen)
O'Brennan (MacBrennan)
MacBride (Kilbride)
O'Brien
Browne
O'Buhilly (Buckley)
Burke (Bourke, de Burgh)
Butler
O'Byrne
MacCabe
MacCafrey (MacCafferky)
O'Cahill
O'Callaghan (Kelaghan)
MacCann (Canny)
O'Cannon (MacCannon)
Carey (O'Keary, Carr)
O'Carolan (Carleton)
O'Carroll (MacCarroll, MacCarvill)
MacCartan (Carton)
MacCarthy
O'Casey (MacCasey)
O'Cassidy
MacClancy (Glanchy)
O'Clery (Clarke, MacClery, Clerkin)
O'Coffey (Cowhig)
MacCoghlan (O'Coughlan, Cohalan)
MacColgan
O'Colman (Coleman, Clifford)
O'Concannon
Condon
O'Connell (Gunning)
O'Connellan (Conlan, Conlon)
O'Connolly
O'Connor
O'Conry (Conroy, O'Mulconry, King)
MacConsidine
MacConway (O'Conway, Conboy, Convey)
O'Cooney
O'Corcoran (MacCorcoran)
MacCormack (O'Cormacan)
O'Corrigan (Carrigan)
MacCostello (Nangle)
MacCotter
MacCoy (MacCooey)
Coyne (Kilcoyne, Kyne, Coen)
Creagh
O'Crean (Crehan, Cregan)
O'Crowley
O'Cullane (Collins)
O'Cullen
O'Cullinan (Quillinane)
Cummins (Comyn, Commons)
O'Cunigan (MacCunigan, Cunningham, Dongan, Counihan)
MacCunneen (O'Cunneen, Kenyon, Kinnane)
MacCurley (Kerley, Turley, Terry)
O'Curran
O'Curry (Corry)
MacCurtin
Cusack
Dalton
O'Daly
Darcy (O'Dorcey, MacDarcy)
O'Dargan (Dorgan)
O'Davoren
O'Dea
O'Delany (Delane, Dillane)
O'Dempsey
MacDermot (Kermode)
O'Devine (Davin, Devane)
O'Devlin
Dillon
O'Dinneen (Downing)
O'Doherty (MacDevitt)
O'Dolan (Doolan)
MacDonlevy (Dunleavy, Leavy)
MacDonnell
O'Donnell
O'Donnellan (Donlon)
O'Donnelly
MacDonogh (Donaghy)
O'Donoghue (Donohoe, Dunphy)
O'Donovan
O'Dooley
O'Doran (Dorrian)
O'Dowd (Dowda, Doody, Duddy)
O'Dowling
O'Downey (MacEldowney)
Doyle (MacDowell)
O'Driscoll
O'Duffy (Duhig, Dowey)
O'Duggan (O'Dugan)
O'Dunn (Dunn)
O'Dwyer
MacEgan (Keegan)
MacElroy (MacGilroy, Kilroy)
MacEnchroe (Crowe)
MacEniry (Henry, O'Henry, Fitzhenry)
MacEvoy (MacElwee, MacGilloway, MacVeagh).
Fagan
O'Fahy
O'Fallon (Falloon)
O'Farrell (O'Ferrall)
O'Farrelly (Farley)
O'Feeney
O'Finn (Maginn)
O'Finnegan
FitzGerald (Barron)
FitzGibbon (Gibbons)
Fitzpatrick (Kilpatrick)
O'Flaherty (Laverty)
O'Flanagan
O'Flannery
Fleming
O'Flynn (O'Lynn)
O'Fogarty
O'Foley (MacSharry)
Forde (MacKinnawe, Foran)
Fox
French (de Freyne)
O'Friel.
Gaffney (Caufield, O'Growney, Keveney, MacCarron)
O'Gallagher
O'Galvin
MacGannon
O'Gara (Geary)
MacGarry (Garrihy, O'Hehir, Hare)
O'Garvey (MacGarvey, Garvin)
MacGee
MacGenis (Guinness, Magennis)
MacGeoghegan
MacGeraghty (Gerty)
MacGilfoyle (Powell)
MacGillycuddy (Archdeacon, Cody)
O'Glissane (Gleeson)
MacGorman (O'Gorman)
O'Gormley (Grehan Grimes)
MacGovern (Magauran)
MacGowan (O'Gowan, Smith, MacGuane)
O'Grady
MacGrath
O'Griffy (Griffin, Griffith)
MacGuire (Maguire).
Hackett
O'Hagan (Aiken)
MacHale
O'Halloran
O'Hanlon
O'Hanly
O'Hannon (Hanneen)
O'Hanrahan (Hourihane)
O'Hanraghty (Enright, Hanvey)
O'Hara
Harrington (O'Harraughton)
O'Hart
O'Hartigan
O'Hea (Hayes, Hughes)
O'Healy (Hely, Kerrisk)
O'Heffernan
O'Hegarty
O'Hennessy (Henchy)
O'Heyne (Hynes)
O'Hickey
O'Higgin
O'Hogan
O'Holohan (Holland, Mulholland, Hyland)
O'Horan (Haren)
O'Hosey (Hussey)
O'Houneen (MacGlashan, Greene)
MacHugh
O'Hurley (O'Murhila, Murrily, O'Herlihy).
MacInerney (Nerney, Kinnerk).
Jennings
Jordan
Joyce.
Kavanagh (Keevan)
Keane (O'Kane, O'Cahan, MacCloskey, MacEvinney)
O'Kearney (Carney)
Keating (Keaty)
O'Keeffe
O'Keenan
O'Kelleher
O'Kelly (Queally, O'Keily)
MacKenna
O'Kennedy (Minnagh)
O'Kenny
MacKeogh (Kehoe, O'Hoey, Hoy)
MacKeon (MacKeown, Owens, Hone)
O'Kieran (O'Kerin, Kearns, Kerrane)
O'Kinneally (Quinnelly)
Kinsella
O'Kirwan.
Lacy (de Lacy)
Lally (O'Mulally)
O'Lalor (Lawlor)
O'Leary
O'Lee (MacAlee)
O'Lenaghan (Linehan)
O'Lennon (Linnane, Leonard, Linnegar, MacAlinion)
O'Lonergan
O'Lorcan (Larkin)
O'Loughlin (Loughnane)
MacLoughlin
O'Lowry (Lavery)
Lynch
O'Lyne (Lyons, Lehane, Lane)
MacLysaght (MacAleese).
O'Madden (MacAvaddy, Madigan)
MacMahon (Mohan, Vaughan)
O'Mahony
O'Malley
O'Malone
Mangan (O'Mongan)
Mannion (O'Mannin, Manning)
Mannix (O'Mannis, O'Manihan, MacNeice)
MacManus
Martin (Gilmartin)
O'Meagher (Maher)
O'Meara (O'Mara)
O'Meehan (Meighan)
O'Molloy (Miley)
O'Molony (Maloughney)
O'Monahan (Monks)
O'Mooney (Meeny, Mainey)
O'Moran
O'More (Moore)
O'Moriarty (Murtagh)
O'Moroney (Mulrooney)
Morris (Fitzmaurice, Morrison)
Morrissey
O'Mulcahy (Muckley)
O'Mullan (O'Mellan, Mullen)
O'Mulligan
O'Mulvihil (Melville, Mitchell)
O'Murphy (O'Morchoe)
MacMurrough
O'Murry (MacMurry, Murray, MacMorrow, Gilmore).
O'Naghten (MacNaughton)
Nagle (Nangle)
MacNally (MacAnally)
MacNamara
O'Neilan (Neylan)
O'Neill (Nihill)
O'Nolan
Nugent (Gilsenan)
MacNulty
O'Nunan (Noonan).
O'Phelan (Whelan)
Plunkett
Power
Prendergast (Pender)
Purcell.
MacQuaid
O'Quigley (Cogley)
MacQuillan
MacQuilly (Cox)
O'Quinlan (Quinlevan, Kindellan)
O'Quinn.
O'Rafferty (Roarty, Raftery)
MacRannall (Reynolds, Grannell)
Redmond
O'Regan
O'Reilly (O'Rahilly)
Rice (O'Mulcreevy)
O'Riordan (Rearden)
Roche (Rochfort)
O'Rooney
MacRory (Rogers, MacCrory)
O'Rourke
Ryan (O'Mulrian).
Sarsfield
O'Scanlan (O'Scannell)
O'Scully
O'Shannon (O'Shanahan, Gilshenan, Giltenan)
MacShanly
O'Shaughnessy
O'Shea (Shee)
O'Sheehan (Sheahan)
MacSheehy
O'Sheridan
O'Shiel (Shields)
Skerrett
O'Sullivan
MacSweeney.
Taaffe
MacTiernan (MacKiernan)
O'Tierney (Tiernan)
Tobin
O'Toole (Toal)
O'Tracy
Traynor
O'Trehy (Troy)
Tully (MacAtilla, Flood)
O'Twomey.
Wall
Walsh (Brannagh)
MacWard (Ward)
Woulfe