He was born in Inis Mór, County Galway. His parents were national school teachers, Pádraic Ó hEithir and Delia Ní Fhlaithearta. He attended their school in Kilronan. He received his secondary school education at Coláiste Éinde (St Enda's College), Galway. He attended University College Galway for three years but left without obtaining a degree. Ó hEithir wrote in both Irish and English, and was highly regarded for the originality and liveliness of his journalism, especially his work in Irish.
He married Catherine von Hildebrand in 1957 and they had five children; Ruairí, Máirín, Brian, Aindriú, and Rónán. Catherine was 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. He was a nephew of Aran Islands authors Liam Ó Flaithearta and Tom Maidhc Ó Flaithearta, the brothers of his mother, Delia. A biography of Ó hEithir has been written by Liam Mac Con Iomaire.