The Critics's Circle website says that anyone who comes from the British Isles (therefore UK + Ireland) are eligible for the award. It does sound funny, but why refuse such a critical accolade all in the name of semantics? He played Bobby Sands, the face of IRA! Isn't that nationalistic enough for that boob? Plus, Fass can say: my mum is Northern Irish, so I technically am a Brit (not that he ever would say that). No kidding, he actually could have had a british passport, were he born after 1983, because Ireland's laws re: nationality are jus sanguinis. I might be totally wrong here, in which case, please feel free to correct me!
He might actually have a British pasport. His mother might still have hers as well. Who knows?