There is also the option of travelling by coach rather than flying. Eurolines do a coach into Belfast. I know they go from London and Manchester - you'd have to look on their website to see if they go from another station close to you (as I don't know where abouts you're based). I know when I looked between Manchester/Belfast it would cost £50 - that's 3x less than what the airfare is going to cost.
I'd be coming from Devon most likely, and I looked at flights from Bristol to Belfast and they were around £27 each way. That was with EasyJet though, and I only know what I've heard I can imagine a coach taking a very long time, but I'll definitely look into it as an alternative, thank you