O'Rahilly, Thomas Francis (‘T. F.’) (1882–1953), scholar, was born 11 November 1882 in Listowel, Co. Kerry, seventh among fifteen surviving children of Thomas Francis Rahilly of Ballylongford, businessman and later court clerk, and Julia Mary Rahilly (née Curry) of Glin, Co. Limerick. An elder brother, Alfred (qv), had a distinguished career in UCC, and a sister, Cecile (qv), became a leading Celtic scholar. Thomas Francis was educated at St Michael's College, Listowel, and Blackrock College, Dublin, and UCD, graduating from the Royal University with a BA in Irish and classics (1905). Having held a temporary post in Irish at UCD (1905–6) he took up a permanent position as a clerk in the Four Courts in 1906.
His part-time studies in subsequent years were so productive that he came to be regarded as the foremost scholar of Modern Irish. A subsidiary interest was the simplification of Irish spelling, and he served as secretary of the Cumann um Letiriú Shimplí in 1910. In 1912 he founded and edited a journal, Gadelica, devoted to the study of modern Irish language and literature since the seventeenth century, but only one volume appeared owing to a shortage of subscriptions. In 1916 he completed an MA thesis, ‘The accentuation of Gaelic’, and published Dánta grádha: an anthology of Irish love poetry. Nevertheless, he was unsuccessful in his application for the chair of Modern Irish at UCC later that year, despite the support of many leading scholars of Irish and Celtic studies. In 1918 he married Mary Buckley from Rathcormack, Co. Cork; they had no family. O'Rahilly had lodged at 66 Botanic Road, Glasnevin, since 1901 but he and his wife now took up residence at 4 Highfield Road, Rathgar. He became professor of Modern Irish at TCD in 1919 and subsequently held the positions of NUI research professor in Gaelic languages (at UCC 1929–35, at UCD 1935–40); senior professor in the School of Celtic Studies at the Dublin Institute for Advanced Studies (1940–48; director of the School, 1941–7); and honorary professor of Irish at TCD (1952–3). During the 1920s O'Rahilly published a succession of textual editions, mostly of early modern poetry: Dánfhocail: Irish epigrams in verse (1921), A miscellany of Irish proverbs (1922), Burdúin bheaga: pithy Irish quatrains (1925), Laoithe cumainn (1925), Dánta grádha: an anthology of Irish love poetry AD 1350–1750 (1925), and Measgra dánta I and II (1927). His principal linguistic work, Irish dialects past and present: with chapters on Scottish Gaelic and Manx, appeared in 1932. Thereafter he worked mainly on Irish historical and cultural origins and published the short works The Goidels and their predecessors (1936) and The two Patricks (1942), and a major work, Early Irish history and mythology (1946). His edition of the counter-reformation text Desiderius (1941) derives from earlier activity. He published many articles, and Ériu, xiii (1942), is largely his work. He served on a committee charged with devising a reformed spelling of Irish for use in the constitution of 1937, but this group failed to agree; however, O'Rahilly's proposals were to influence the postwar spelling reform. As director of the School of Celtic Studies from 1941 he oversaw a project to investigate the spoken dialects of Irish. The latest in a series of disputes led to O'Rahilly's sudden resignation from the directorship of the School in 1947 and his retirement in January 1948.
O'Rahilly died suddenly at home on 16 November 1953. His wife apparently attributed his death to overwork and burned a large quantity of his papers. Those that survived are held at the School of Celtic Studies; his books were bequeathed to QUB. Much of O'Rahilly's work on early Ireland has been superseded or discounted, and his considerable reputation rests mainly on his editions and linguistic studies, especially Irish dialects past and present. He received the honorary degrees of D.Litt.Celt. from the NUI (1928) and D.Litt. from TCD (1948).