Actually he had been an air steward with Ryanair for 4 years, so presumably he'd been living in the UK for that length of time. Incidentally, he was in a restaurant in Bishop's Stortford when he got the call to tell him he was in, not Ireland.
I've never heard that BB was only open to UK nationals. When you think about it, there have been quite a few who weren't. Anna, Brian, Amma, Vanessa, Nadia, Ahmed ... all of them born outside the UK, but living here now. Those are just off the top of my head, there are probably more.
Never actually thought of it as an issue, since "it's only a gameshow" not the Olympics!