I've always heard FlyBE struggle to keep pilots full stop but that Belfast City is one in particular.I've held long aspiratiosn to get a base at City having worked there for 3 years!
Just awaiting them re-opening their online application.
Yes Jet.2 and Easyjet both have bases in Northern Ireland and as far as I'm aware so do Mytravel and First Choice(correct me if I'm wrong) though these are limited in size.
Regards
P.S. Good luck in your studies