Declan was born in Donegal in north west Ireland and has been writing since he was a young child. His film career began when he was cast in John Sayle's The Secret of Roan Inish in 1993 and has continued since in films such as The Martian and Atomic Blonde. He played the lead antagonist in the biopic Curtiz about the making of Casablanca. He joined the cast of The Terror for AMC and featured in The Witcher. Declan has also written and directed for the screen, winning the Madrid Film Festival with his short Csapas in 2008 and winning the inaugural Hypewriter contest with his script The Butcher.