I don't know, maybe a Norweigan 737?
With United now bringing a 777-200 from EWR and a 767-300 from ORD, there is pretty much only 1 or 2 routes from Dublin which are now operated by narrowbody aircraft.
It used to be that around 40-50% of the transatlantic flights were operated by 757-200. This change in seat capacity shows how much Dublin has grown. I guess Dublin has pretty much taken up most of the demand that existed from Northern Ireland.
A 737 sized aircraft (150 seats) is the most that I can see being feasible from Belfast. I don't know what airline would do this other than Norwegian.