Dervla Kirwan played Mercy Hartigan in The Next Doctor.
Though Irish, her career has mostly been in the British market, where she has amassed a long string of starring television roles. She has been a part of the regular casts of several British programmes like Goodnight Sweetheart, Eureka Street, Hearts and Bones, 55 Degrees North, and True Dare Kiss. She is likely most famous, however, for her National Television Award-winning role of Assumpta Fitzgerald in Ballykissangel, a role with which she crossed over into both The Vicar of Dibley and Father Ted.
She has been in a number of productions alongside other key Doctor Who actors. In one of her first major television serials, A Time To Dance, she was joined by guest stars David Banks and Geoffrey Beevers. In 1999 she played Christopher Eccleston's wife in the film With or Without You. In 2001, she and Derek Jacobi guest-starred in an episode of Randall & Hopkirk (Deceased) alongside regular cast member Tom Baker. In Hearts and Bones, she was a regular cast member with fellow Christmastime baddie Sarah Parish. Like David Tennant and Shaun Parkes before her, she came to Doctor Who after an appearance in the Russell T Davies/Julie Gardner production of Casanova. Though she did not have any scenes with Tennant in Casanova, she did in the one-off 2004 production, The Deputy.
After her Doctor Who appearance, she guest-starred in a couple of episodes of Law & Order: UK, alongside Freema Agyeman and Bill Paterson. She also made the Irish film Ondine with Tony Curran. In 2010, she starred alongside Lenora Crichlow in the series Material Girl.