Ó Dónaill is the olderst of the six children of Tarlach Ó Dónaill and Éilis Nic Ruairí from Grial, Loughanure. They own a little land and a few cows. His father spends June to November working in Scotland and dies when he is 13 years old. He himself spends summers working in the tunnels in Scotland.
Ó Dónaill receives his education at Scoil Loch an Iúir in Loughanure before gaining a scholarship to St. Eunan’s College in Letterkenny. Another scholarship takes him to University College Dublin to study Irish, English and History. During his time in university Ó Dónaill spends his summers teaching at Coláiste Bhríde in Rann na Feirste, County Donegal.
Ó Dónaill writes the book Bruigheann Féile which is based on stories of pastimes in the Gaeltacht town Loughanure and its surrounding area. His book Na Glúnta Rosannacha is first published in 1952.
Ó Dónaill is most famous for his work as editor of the 1977 Irish-English dictionary Foclóir Gaeilge-Béarla, which is still widely used today.
Ó Dónaill dies in Dublin on February 10, 1995.