Kirwan has been married to actor Rupert Penry-Jones since 2007, after a four-year engagement. They are parents to two children, Florence and Peter. They met in 2001 while working on stage together in a West Yorkshire Playhouse, Leeds, production of JB Priestley's "Dangerous Corner", when he played Robert Caplan to her Olwen Peel. Kirwan again appeared on stage with Penry-Jones in "Les Liaisons Dangereuses" in 2003. They also both appeared in the television show "Casanova" in 2005, although they did not share any scenes.