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 Good, The Martian and Atomic Blonde. He appeared in The Witcher on Netflix and The Terror a horror series for AMC.
Declan has also written and directed for the screen, winning the Madrid Film Festival with his short Csapas in 2008. He has written for AMC and RTL Hungary. With Andy Hefler, Declan won the inaugural Hypewriter contest and from that made the TV pilot for their series The Butcher.