O'Neill presents a very complex multi-layered kind of challenge. His characters are always deeply complex and, to a great extent, inaccessible.

He played football in Dublin with the Stella Maris Football Club in Drumcondra. He also received the Honorary Patronage of the University Philosophical Society of Trinity College Dublin on February 20 2007. [citation needed]
In January 2011 he spoke in an interview on The Meaning of Life about being sexually abused by priests during his childhood.

His acting career began in the Focus Theatre before he joined London's Royal Court Theatre in 1979. Byrne's screen debut came in the Irish soap opera The Riordans and the spin-off show Bracken.