Thomas Francis O'Rahilly (Irish: Tomás Proinsias Ó Rathaile; 1883–1953) was an Irish scholar of the Celtic languages, particularly in the fields of Historical linguistics and Irish dialects. He was a member of the Royal Irish Academy.
He was born in Listowel, County Kerry, Ireland. Educated at the Royal University of Ireland, he held professorships in Irish at Trinity College, Dublin (1919-1929), and in Celtic languages at University College, Cork (1929-1935), and University College, Dublin (1935-1941). He was director of the School of Celtic Studies at the Dublin Institute for Advanced Studies from 1942 to 1947.
O'Rahilly was known for his sometimes controversial theories of Irish history. In his book Early Irish history and mythology, first published in 1946, O'Rahilly developed a model of Irish prehistory based on critical reading of early Irish literary sources, involving four waves of Celtic-speaking invaders (see Early history of Ireland). In a lecture published in 1942 he proposed that there were two Saint Patricks.
His views on language contact and bilingualism were equally controversial. In Irish dialects past and present (1932) he wrote the following about Manx:
From the beginning of its career as a written language English influence played havoc with its syntax, and it could be said without much exaggeration that some of the Manx that has been printed is merely English disguised in a Manx vocabulary. Manx hardly deserved to live. When a language surrenders itself to foreign idiom, and when all its speakers become bilingual, the penalty is death. (p. 121)
This view has more recently been challenged by Nicholas Williams, who suggests that Manx is Gaelic pidginized by early contact with Norse, long before there was any English spoken in Man.
Other publications include a series of anthologies of Irish verse published between 1916 and 1927. He founded and edited Gadelica: a Journal of Modern Irish studies, and edited the journal Celtica (1946-1950).
His sister Cecile O'Rahilly was also a Celtic scholar, and published editions of both recensions of the Táin Bó Cúailnge.
He was a brother of Alfred O'Rahilly a noted academic, President of University College Cork and Teachta Dála (TD) for Cork City.
He died in Dublin.