Belfast is relatively straightforward also for onward / return travel to Dublin if flight prices are crazy.
On the contrary, I'd say it's a bit of a pain in the bollocks.
Fly from wherever to Belfast International. Have to get a 40 minute bus to Europa. Then get a train to either Lanyon Place or Lisburn/Portadown to get on the Enterprise to Dublin which is only every 90 minutes or 2 hours and sit on the train for a couple of hours.
A quick flight direct to Dublin suddenly becomes a 4 or 5 hour journey if you go via Belfast.
Flying into the City would eliminate the 40 minute bus journey, but you're then having to get a taxi from there to Lanyon for the train. c*nts would probably charge £20 for a 5 minute drive.
The bus service doesn't seem to be much better. I don't see anything on Translink website with any special services for it.