The thing to remember is that most 757s that fly across the pond are in a mixed class config. All the local airlines that fly the 757 (excluding EI) are leisure airlines in a high seating config. These aircraft tend to have less capability for range. Also remember Canada has no 757s in passenger service (excluding TCX chartered to Westjet), so this leaves the likes of Air Canada Rouge or Air Transat to provide services to Toronto (Westjet cannot fly direct)
In the past charter airlines were very keen to use the 767 in Belfast as they were multi sector flights that tended to start in the Mainland. Nowadays if you want to fly direct airlines have to operate positioning flights, or W-patterns, then transport crew across..all this is logistically more complicated and airlines will be less keen to do this on small or low yielding markets, such as Northern Ireland.
On a latter note about 737s operating transatlantic, for the likes of Boston or New York yes it is theoretically possible, but the weight restrictions would be substantial, and as a result probably non-profitable. Owenc if you don't believe me I recommend you fly Oslo-Dubai with Norwegian on the 737 some time, you'll have almost 7 hours to count how many empty seats are onboard. That's without worrying about ETOPS too.