O'Faodhagáin- Fagan - E. Airgialla (Oriel) - also of Norman origin
Ó Fathaigh - Fahy - Co Galway - Uí Máine, of Pobal Muintir Uí Fhathaigh
Ó Fallamháin - (O) Fallon - Co Roscommon - sept of the Uí Máine (Clann Udach)
Ó Fallamháin - (O) Fallon - Co Meath - of Crich na Cetach
Ó Failbhe - (O) Falvey - Co Kerry - the Corca Duibhne
Ó Fhearghaill or Fergail - (O) Farrell - Co Longford - lords of Annaly
Ó Faircheallaigh - (O) Farrelly or Farley - Co Cavan/Meath
Ó Faracháin - (O) Farren - Co Donegal - Lord of Finross
Ó Fearáin - (O) Fearon - Co Armagh (Oriel) - the Cenél Eoghain
Ó Fiannaidhe - (O) Feeney - Co Sligo/Mayo - sept of the Uí Fiachrach
Ó Fidhne - (O) Feeney - Co Galway/Roscommon
Ó Fithcheallaigh - (O) Fehilly, Field - Co Cork - sept of the Corca Laoidhe
Ó Fiachna - - (O) Feighney, Feeheny, Hunt - Co Roscommon/Sligo - of Síol Muireadhaigh
Ó Fiachra - - (O) Feighry, Feehery, Hunt - Co Tyrone
Ó Fionnaláin - (O) Fenelon - Co Westmeath - chiefs of Delvin (Delbna)
Ó Fiangusa - (O) Fennessy - Co Tipperary/Waterford - chiefs of a district near Cashel
Ó Fearadhaigh - (O) Ferry, Farry - Co Donegal - Cenél Conaill
Ó Fionnachta - (O) Finaghty, Finerty - Co Roscommon - of Síol Muireadhaigh
Ó Fionnachta - (O) Finaghty, Finerty - Co Galway/Roscommon - Uí Maine chiefs of
Clanconway
Ó Fionnáin - (O) Finan - Co Mayo - Uí Fiachrach Muaighe chiefs of Coolcarney
Ó Fionn - (O) Finn - Co Sligo - chiefs of Calry
Ó Fionn - (O) Finn - Co Galway - erenaghs of Kilcolgan
Ó Fionn - (O) Finn - Co Monaghan - of Fir Rois
Ó Fionnagáin - Finnegan - Roscommon/Galway
Ó Fionnagáin - Finnegan - Cavan/Monaghan/Louth - a Breifne sept
Ó Finntighearn - Finneran - Co Westmeath? - chiefs of Uí Mella
Mac Fionbhair - Finnevar - Co Leitrim - chiefs of Muintir Greadain
Mac Giolla Mo-Cholmog - Fitz Dermot - Uí Dunchada of Dublin/Kildare

Mac Giolla Phadraig - Fitzpatrick - lords of Upper Ossory (Laois/Kilkenny)
Ó Flaithbheartach - (O) Flaherty - Co Galway - Lord of Moycullen and Iar-Chonnacht
Ó Flannagáin - Flanagan - Roscommon & Offaly
Ó Flannagáin - (O) Flanagan - Fermanagh - Cenel Cairbre of Tuath Ratha (Toora)
Ó Flannagáin - (O) Flanagan - Monaghan - Clan Ceallaigh - the MacDonnels of Clankelly
Ó Flannabhra - Flannery - Co Mayo - the Uí Fiachrach
Ó Flannabhra - Flannery - Co Limerick - the Uí Fidhgheinte, chief of Dal Cairbri
Ó Flaithre - (O) Flattery - Co Down - lords of Lecale, kings of Ulidia
Ó Floinn - (O) Flynn - Co Cork - Muscraige Uí Fhloinn
Ó Floinn - (O) Flynn - Co Cork - Corcu Laoidhe
Ó Floinn - (O) Flynn - Co Roscommon
Ó Flainn - (O) Flynn - Co Antrim
Ó Fhloinn - (O) Lynn - Co Armagh - Clanna Rury, descent from Coll Uais
Ó Fhógartaigh- Fogarty - Co Tipperary, Eliogarty - Dalcassian
Ó Foghladha - Foley - Co Waterford
Mac Giolla na Naomh - Ford - South Connacht
Mac Conshnámha - Forde or MacKinnawe - Co Leitrim - chiefs of Muintir Kenny
Ó Fuaráin - Forde or Foran - Co Cork
Macgiollarnath - Ford - southern Connemara
Ó or Mac an tSionnaigh - Fox - Co. Meath - nickname of a chief of Teffia
Ó Firghil - (O) Friel - Co Donegal - descent from Eoghan, brother of St. Columbcille