To be honest, yes QUB can be quite cliquey (this is true of Northern Ireland in general). Look, there are English people that come over from GB to QUB and make friends etc, but I think it's harder to make friends at QUB than at a typical English uni. Lots of QUB students go home to their parents' houses at the weekends, for example. And lots of QUB students will already have groups of friends from their hometown/school etc, so they may not be as keen to make new friends as in your average English uni.
Disclaimer: This is only my personal opinion!