Killian Donnelly has appeared in many different roles around Ireland and has also directed and written shows. His first theatre role was in 2005 where he featured in the chorus of The Wireman at the Gaiety Theatre, Dublin. He then continued to play Rod in Singin' in the Rain and Ethan in The Full Monty, both at the Olympia Theatre, Tony in West Side Story at the Solstice Theatre, Aladdin in Aladdin and Collins in Michael Collins: A Musical Drama, both at the Cork Opera House, Prince Charming in Cinderella at the Gaiety Theatre, Dublin and Chorus in Sweeney Todd: The Demon Barber of Fleet Street at the Gate Theatre.

In 2008, he joined the West End production of Les Misérables as a swing and, as a member of the cast, was part of the Les Misérables concert with the Bournemouth Symphony Orchestra on the Isle of Wight. He became second cover Javert and Enjolras in the following season of 2009/10, during which time he also performed the lead role of Jean Valjean as an emergency cover multiple times. With the following cast change he was promoted to principal Enjolras and was officially listed (and performed) as third cover Valjean. He eventually finished in Les Misérables on 18 June 2011.

Donnelly's next role was Raoul in Andrew Lloyd Webber's The Phantom of the Opera at Her Majesty's Theatre. During this time, he also featured in commercials for McDonald's and Jameson Whiskey, and appeared on such television shows as The Tudors, The Open House, The Late Late Show and Showbands. He later performed in You Should Be Dancing at the National Concert Hall.

On 3 October 2010, Donnelly played the role of Courfeyrac in the 25th Anniversary Concert of Les Misérables at The O2 Arena in London. He also played the role of Combeferre in the film of the show, released in 2012.

On 12 November 2012, Donnelly took over the role of Tony in Billy Elliot The Musical at the Victoria Palace Theatre.

On 20 August 2013, it was confirmed that Donnelly would originate the role of Deco in the West End Musical The Commitments from 21 September. The following year Donnelly began playing the role of Huey in Memphis from 9 October 2014 for which he was nominated as Best Actor in a Musical in the 2015 Olivier Awards

On 23 April 2015, it was announced that Donnelly would play Charlie in the West End production of the musical Kinky Boots, at the Adelphi Theatre. The first preview was on 21 August 2015. On 29 February 2016 Donnelly was nominated as Best Actor in a Musical at the 2016 Olivier Awards. He left the show on 13 August 2016 and moved back to Ireland to play Jackie Day in Donegal, a new play by Frank McGuinness.

On 6 December 2016, he made his Broadway debut, reprising the role of Charlie Price in Kinky Boots.

On 12 June 2017, Donnelly returned to the West End production of Les Misérables to play the lead role, Jean Valjean, subsequently reprising the role in the 2018 tour in the United Kingdom and Ireland.

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