[citation needed] Pre voice-breaking parts included Soothsayer in Julius Caesar, Duchess of York in Richard III, and Maisie Madigan in Juno and the Paycock, followed by McLoghlain "The Curate" in Joseph Tomelty's Is the Priest at Home and Seamus Sheilds in The Shadow of a Gunman.
[citation needed] In his final year at Terenure College in 1973, he was spotted by Joe Dowling playing Donald in a production of Simon Gray's Spoiled at the Project Arts Centre.
[citation needed] He played David Charters in SOS Titanic (EMI Films) and made two major TV series for RTE, The Burke Enigma, a drama produced and directed by Brian Mac Lochlainn, and Seán, a thirteen-hour drama series on the early life of Sean O'Casey that was produced and directed by Louis Lentin.
[citation needed] Plays and musicals included Mary Makebelieve,The Lugnaquilla Gorilla, Hamlet, The Plough and the Stars, The Hidden Curriculum, Childish Things, Fiche Blían ag Fás, Petty Sessions, The Merchant of Venice, The Glass Menagerie and The Silver Dollar Boys.
[citation needed] In 1983, he worked again for RTE playing Sweeney in Eagles and Trumpets (directed by Deirdre Friel) and for Mobile Showcase Theatre USA, as Tarquin in A Careful Man by Frederick Forsythe.
[citation needed] He continued appearances at the Abbey Theatre and the Peacock, Manus in The Hard Life by Flann O'Brien in 1986 and Dave in Neil Donnelly's Goodbye Carraroe in 1989.
[citation needed] In 2004, he was engaged by Ouroboros Theatre Company as Baron van Sweeten in Amadeus, which subsequently led on to him playing Peter Lombard in Geoff Gould's production of Brian Friel's Making History in 2006 and 2007.
[citation needed] This was a record-breaking production that opened in the Samuel Beckett Theatre Dublin and played all over Ireland in various site-specific venues that were made available by the Office of Public Works.
[citation needed] In 2007, the 400th anniversary of The Flight of The Earls, the production also toured to several cities in France and Switzerland following the route of Hugh O'Neill's trek across Europe and finally to Rome.
[citation needed] Ouroboros were subsequently invited to perform the play for Brian Friel at the celebrations for his 18th birthday at the Magill Summer School, Glenties County Donegal in July 2009.
[citation needed] The play premiered initially at the Dublin Fringe Festival in the Project Arts Centre receiving rave reviews.
[citation needed] Also in 2007, Landmark produced the huge hit The Last Days of the Celtic Tiger in the Olympia Theatre by Paul Howard, a satire on Ireland's 'get rich quick' years leading up to "The Crash" of 2008.
[citation needed] Paul Howard wrote and Landmark produced two further Ross satires, Between Foxrock And A Hard Place in 2010 and 2011, and Breaking Dad in 2014 and 2015 with O'Sullivan reprising his role as Charles.
[citation needed] In 2013, he began filming on Vikings as the recurring character Bishop Edmund, advisor to King Ekbert played by his friend Linus Roache.
[citation needed] Towards the end of 2016, O'Sullivan was cast in Mel Gibson's film The Professor and The Madman, as the prosecuting Queen's Counsel to Sean Penn's accused and mad Dr.