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 The Coldest City. Declan has also written and directed for the screen, winning the Madrid Film Festival with his short Csapas in 2008. Declan lives in Hungary.