Ó hEithir’s parents are national school teachers, Pádraic Ó hEithir and Delia Ní Fhlaithearta. He is a nephew of Aran Islands authors Liam Ó Flaithearta and Tom Maidhc Ó Flaithearta, the brothers of his mother. He attends the Kilronan national school where his parents teach. He receives his secondary school education at Coláiste Éinde (St. Enda’s College), Galway. He attends University College Galway for three years, finishing his university course in 1952 but leaves without sitting his final examinations. He writes in both Irish and English, and is highly regarded for the originality and liveliness of his journalism, especially his work in Irish.
Ó hEithir marries Catherine von Hildebrand, a young student recently arrived in Dublin from Colombia, in 1957 and they have five children: Ruairí, Máirín, Brian, Aindriú, and Rónán. Catherine is born in Paris, the daughter of Deirdre Mulcahy from Sligo and Franz von Hildebrand from Munich, son of the noted philosopher and theologian Dietrich von Hildebrand.
After college, Ó hEithir spends a number of years working as an itinerant bookseller for Comhdháil Náisiúnta na Gaeilge. He serves as an editor at Sáirséal agus Dill, the Irish language publishing house, and as Irish language editor for The Irish Press from 1957 to 1963. He also writes a column for The Sunday Press. He is a regular columnist with the journal Comhar and also contributes a weekly column to The Irish Times. He also serves as a staff journalist with Raidió Teilifís Éireann (RTÉ), working on the current affairs programmes Cúrsaí and Féach.
In 1975 the Irish American Cultural Institute awards Ó hEithir a scholarship of £2,000 to allow him to devote more time to writing. The following year his first novel, Lig sinn i gcathú (1976), loosely based on his student days in Galway, becomes a best-seller. He and Catherine move to Paris in 1986, where most of his second novel, Sionnach ar mo Dhuán (1988), is written. Hopes of having produced his definitive novel are soon dashed by a series of devastating reviews.
Ó hEithir visits Colombia with his wife in the summer of 1990. On his return, he is presented with the Butler literary award of $10,000 in further recognition of his writing in Irish. A month later, after a very short illness, he dies of cancer in St. Vincent’s University Hospital in Dublin on October 26, 1990. He is survived by his wife, Catherine, daughter Máirín, and sons Ruairí, Brian, and Aindriú.
A biography of Ó hEithir has been written by Liam Mac Con Iomaire.