From Wikipedia -
Dudi Appleton, director, screenwriter and journalist
Paul Bew, scholar and life peer
Derek Bell, harpist, late member of The Chieftains
Andrew Bree, swimmer
Gordon Burns, journalist and television presenter
Sir Anthony Campbell, retired judge
Ben Clarke, The Apprentice candidate in 2009
Sir John Collins, businessman
David Crawford Librarian
Freeman Wills Crofts, author; was a member of the school's first class in 1894
George Currie, British Member of Parliament
Eric Robertson Dodds, classical scholar
William John English, Victoria Cross recipient whose medal was bequeathed to the school
Thomas Henry Flewett, virologist
Mike Gibson (rugby player)
Paddy Hirsch, journalist, award-winning Marketplace radio producer and presenter
Michael Hoey (golfer)
John Irvine, award-winning ITV News journalist
Charles Lawson, actor
C.S. Lewis, Author
Gary Lightbody, vocalist and guitarist in Snow Patrol
William MacQuitty, film producer
James Godfrey MacManaway, MP and Church of Ireland minister
Tim Martin, founder and current Chairman of JD Wetherspoon
Sir John MacDermott, Baron MacDermott, former Lord Chief Justice of Northern Ireland
Alan McFarland, former British Army officer and Ulster Unionist politician
Sir Percy McElwaine, barrister and Attorney General of Fiji
Colonel Sir Michael McCorkell - Northern Irish soldier
Alan McKibbin, British Member of Parliament
John Morrow, peace activist
Jonny Quinn, drummer in Snow Patrol
Robert Shanks, former Chairman of the Commissioners of Irish Lights and commanding officer of HMS Caroline
James Simmons, poet
Air Chief Marshal John Thomson, RAF officer
Lloyd Hall-Thompson, British Member of Parliament
Noel Thompson, BBC journalist
Chris Farrell, rugby union footballer for Ulster U18s, Ulster U20s, Ulster and Ireland U18s
Jamie Smith, rugby union footballer for Ulster
Hong-Wing Yeung, Businessman
Paddy Wallace, rugby union footballer for Ireland
Edmund De Wind, Victoria Cross recipient
C.S. Lewis is a bit rich though, think he was only there for one term and hated it because he was bullied.
Samuel Beckett taught for a couple of terms at the school where apparently he was told that he was teaching the cream of Ulster. "Yes," he replied. "Rich and thick."