Byrne, the first of six children, was born in Crumlin, Dublin, Ireland, the son of a cooper and soldier, Dan, and a hospital nurse from Galway, Eileen (née Gannon). His siblings are Donal, Thomas, Breda, Margaret. Another, Marian, died at a young age. Byrne was raised Catholic and educated by the Irish Christian Brothers. With regard to his early training to become a priest, he said in an interview, "I spent five years in the seminary and I suppose it was assumed that you had a vocation. I have realised subsequently that I didn't have one at all." He attended University College Dublin, where he studied archaeology and linguistics, becoming proficient in Irish. He played football in Dublin with the Stella Maris Football Club in Drumcondra.