Damian was born in Sutton Coldfield, England, to Irish parents Kate and Patrick Gallagher. He trained at The Oxford School of Drama, which is widely classed as one of the most prestigious drama schools in the United Kingdom. He has appeared in films such as, Avengement, alongside Scott Adkins, Madness in the Method, with Jason Mewes, Kevin Smith, Danny Trejo and Gina Carano, Nandor Fodor and the Talking Mongoose, with Simon Pegg, Minnie Driver and Christopher Lloyd, and the Norwegian cult classic horror film LYST. Prior to returning to drama school, Damian worked in film production, for film producer David Ladd (Hart's War) and alongside David's brother, Oscar winning producer Alan Ladd Jr. (Braveheart). Having spent five years working in Los Angeles, Damian returned to the UK for his drama training, and now splits his time between the UK and America.