Basings are in the gift of the airline. You can't choose to join, say, easyJet on condition that you are based at Belfast. You join and accept whatever basing they give you. Once you have some seniority under your belt, you may have some influence over where you are based. You will, however, be able to express a preference on joining and, if NI is an unpopular base, you may be lucky. But don't base too many plans on it!
Scroggs