Byrne was the first of six children, born in Dublin, Ireland. His father was a cooper and his mother a hospital worker. He was raised Catholic and educated by the Irish Christian Brothers. He spent five years of his childhood in a seminary training to be a Catholic priest. He later said, "I spent five years in the seminary and I suppose it was assumed that you had a vocation. I have realized subsequently that I didn't have one at all. I don't believe in God. But I did believe at the time in this notion that you were being called." He attended University College Dublin, where he studied archeology and linguistics, and became proficient in Irish. He played football (soccer) in Dublin with the Stella Maris Football Club.
Byrne worked in archeology after he left UCD but maintained his love of his language, writing Draíocht (Magic), the first drama in Irish on Ireland's national Irish television station, TG4, in 1996.
He discovered his acting ability as a young adult. Before that he worke