O'Sullivan is one of nine children. His father died the year after his birth. He attended primary and secondary education at Terenure College Dublin. A year after finishing school, he was offered a place in the Abbey Theatre Company, Ireland's National Theatre, one of the youngest actors every to receive a Players contract.
From 1979 to 1981 he worked as a freelance actor, pursing TV and film work and independent theatre opportunities. He then returned to The Abbey. He left in late 1983 with a performance record totaling well over fifty plays, nineteen of which were world premieres and four were musicals.
Since the 1990's and 2000's, O'Sullivan has continued to work on many films, TV dramas, plays, and an extensive recording portfolio. He and a business friend option fictional materials with a view to develop and produce for the television market. He is known for his roles in Tristan & Isolde and The Professor and the Madman. He is also known for voice acting roles and his character driven, distinguishable voice forms the soundtrack for many advertisements.
O'Sullivan is a vocal supporter of actors and theatre folks' rights and work opportunities. He lives with his wife Sandra Ellis and their daughter Jeanne-Marie in Glathule, County Dublin. His hobbies include walking and meeting on Killing Hill, France, drinking wine, good conversation, politics, and history.
- He is great friends with costar Linus Roache.
- He had a guest role on Game of Thrones as Citadel Maestor.