- O'Bierne, Brian Oswald
- O'Brien, Barnabas
- O'Brien, Catherine
- O'Brien, Donough
- O'Brien, Eileen
- O'Brien, Flann
- O'Brien, Nora Connolly
- O'Brolchain, Flaithbhertach
- O'Cahan, Donnell Ballagh
- O'Cahan, Rory Dall
- O Carain, Gilla An Choimhded
- O Casaide, Tomas
- O Cianain, Tadhg
- O Cleirigh, Cuchoigriche
- O Cleirigh, Lughaidh
- O Cleirigh, Michael
- O'Clery, Cucoigcriche
- O'Clery, Michael
- O'Colgain, Sean
- O Conchuir, Tomas
- O'Connor, Daniel
- O'Connor, John
- O Creag, Seamus
- O'Devany, Conor (Cornelius)
- O'Doherty, Cahir
- O Doirnin, Peader
- O Domhnaill, Maghnus
- O'Donnell, Frank Hugh
- O'Donnell, Hugh
- O'Donnell, Hugh Balldearg
- O'Donnell, (Red) Hugh
- O'Donnell, Manus
- O'Donnell, Mary Stuart
- O'Donnell, Niall Garbh
- O'Donnell, Nuala
- O'Donnell, Patrick
- O'Donnell, Peadar
- O'Donnell, Rory
- O'Donovan, Gerald
- O'Duffy, Eoin
- O Duilearga, Seamas
- O'Farrelly, Agnes Mary Winifrede
- O'Farrelly, Feardorcha
- O'Fee, Thomas
- O Fhloinn, Riobard
- O Fiaich, Tomas
- O Gallchobhair, Seamus (Gallagher, James)
- O'Gallagher, Redmond
- O'Gara, Fergal
- Ogilby, David
- O'Glacan, Nial
- O Gnimh, Fearflatha
- O Gormain (MacGormain), Muiris
- O'Gowan, Eric Edward Dorman
- O Grianna, Seamus (pseud. Marie)
- O'Hagan, John
- O'Hagan, John Tullyhogue
- O'Hagan, Mary
- O'Hagan, Thomas
- O'Hanlon, John J.
- O'Hanlon, Redmond
- O'Hara, Helen
- O'Hara, William
|
- O'Hartagan (ua hArtaicain), Cinead
- O'Hempsy (Hempson), Denis
- O hEodhasa, Eochaidh
- O hEodhasa, Giolla-Bhrighde, or Maelbhrighid
- O hUallachain, Colman
- O hUiginn, Tadhg Dall
- O'Hussey, Bonaventure (O hEodhasa, Giolla-Bhrighde or Maelbhrighde)
- O'Hussey, (O hEodhasa), Eochaidh
- Oisin or Ossian
- O'Laverty, James
- O Lochlainn, Domhnall
- O'Malley, Ernest
- O'Malley, Grace
- O Maoil Chonaire, Fearfeasa
- O Meallain, Feardorcha
- O'Mulconry
- O'Neill, Arthur
- O'Neill, Brian MacPhelim
- O'Neill, Conn Bacach
- O'Neill, Daniel c.1612-1664
- O'Neill, Daniel 1920-1974
- O'Neill, Felim
- O'Neill, Flaithbeartach
- O'Neill, George
- O'Neill, Gordon
- O'Neill, Henry
- O'Neill, Hugh d.1230
- O'Neill, Hugh c.1550-1616
- O'Neill, Hugh Dubh
- O'Neill, John 1740-1798
- O'Neill, John 1834-1878
- O'Neill, John Bruce Richard
- O'Neill, Moira
- O'Neill, Nial
- O'Neill, Niall More
- O'Neill, Owen
- O'Neill, Owen Roe
- O'Neill, Phelim 1604-1652
- O'Neill, Phelim d.1709
- O'Neill, Rosa (O'Donnell, nee O'Doherty)
- O'Neill, Seamus
- O'Neill, Shane
- O'Neill, Shane MacBrian
- O'Neill, Terence
- O'Neill, Turlough Luineach
- O'Nolan, Brian
- O'Nuallain, Brian (pseud. Flann O'Brien and Myles Na Gopalween)
- O'Nuallain, Ciaran
- O'Nuallain, Gearoid
- O'Quinn, Jeremiah
- O Raifeartaigh, Tarlach
- O'Reilly, Edmund
- O'Reilly, Edward
- O'Reilly, Hugh
- O'Reilly, Michael
- O'Reilly, Philip MacHugh
- Orr, Andrew
- Orr, James
- Orr, William
- O Searcigh, Seamas
- O'Shannon, Cathal
- O'Sullivan, Mortimer
- O Tighearnain, Poall
|