Killian is originally from Meath, Ireland. Theatre includes: his Broadway debut recreating the role of Charlie Price in Kinky Boots (Al Hirschfeld Theatre, New York), having originated the role in London (Adelphi Theatre) - 2016 Olivier Award nomination for Best Actor in a Musical, 2015 Evening Standard Theatre Awards Shortlist for Best Musical Performance, and, for the Original London Cast Recording, 2017 Grammy Award Nomination for Best Musical Theater Album; Jackie in Frank McGuinness' Donegal (Abbey Theatre, Dublin); Huey Calhoun in Memphis (Shaftesbury Theatre) - 2015 Olivier Award nomination for Best Actor in a Musical; Deco in The Commitments (Palace Theatre) directed by Jamie Lloyd; Tony in Billy Elliot the Musical (Victoria Palace Theatre); Raoul in The Phantom of the Opera (Her Majesty's Theatre); Enjolras in Les Misérables (Queen's Theatre); before which he understudied and played the roles of Jean Valjeanand Javert; Collins in Michael Collins (Cork Opera House); Sweeney Todd (Gate Theatre, Dublin); Ethan Girard in The Full Monty (Olympia Theatre); Tony in West Side Story (The Solstice); Declan in The Wireman (Gaiety Theatre); Rod in Singin' in the Rain (Olympia Theatre).

Workshops include: Dessert Firstby in The Witches (NT Studio) directed by Lyndsey Turner; Edward Lewis in Pretty Woman directed by Jerry Mitchell; Gordon in Local Hero directed by John Crowley; Frankie in Angela's Ashes; Pink in The Wall directed by Simon McBurney; Deco in The Commitments.

Concerts include: West End Musicals (Halle Orchestra/Bridgewater Hall, Manchester);The Phantom of the Opera 25th Anniversary(Royal Albert Hall); Courfeyrac in Les Misérables 25th Anniversary (o2 Arena);You Should Be Dancing (National Concert Hall, Dublin); Les Misérables (Bournemouth Symphony Orchestra/Isle of Wight).

Film and Television includes:The 85th Academy Awards (Dolby Theatre, Los Angeles); Combeferre in Les Misérables (Working Title Films) directed by TomHooper; Showbands (BBC);The Last Furlong (RTE); Batchelors Walk (RTE); The Tudors (BBC);The Late Late Show (RTE); Open House (RTE). Most recently: Killian starred as Jean Valjean in the London production of Les Misérables (Queen's Theatre), returning to the production in which he made his West End debut. Currently: Killian is reprising the role of Jean Valjean touring the U.K. and Ireland in Cameron Mackintosh's new production of Les Misérables