People go to Dublin because they don't have a choice from Belfast, not because they enjoy sitting on a coach for 2.5 hours without a toilet.
And the fact that if you're flying from Dublin to say Heathrow, then onwards to Heathrow, it's much cheaper than flying from Belfast due to the much cheaper taxes.
Eg. last year when I went to Hong Kong in March, BA were charging ~£800 from BHD-LHR-HKG while DUB-LHR-HKG was ~£500 and I imagine a lot of people would like to save ~£300!