The workload on a TFS and a JFK are completely different; on a TFS not every passenger eats/drinks. You have a BOB service and duty free. A T/A service is far busier, and far more labour intensive, hence why every other 757 operator operates with 6 crew. You haven't flown on a T/A service in awhile if you think the service can be done in 2 hours!
The 2 hours wasn't referring to T/A flights. Clearly going for 4 crew is for profitability and having extra crew will eat into potential profits. I suspect the A330 service is very weak if not loss making and they have to get them profitable as closing them just isn't an option.
Also, EI wont be just operating the 757 from Shannon; the new Dublin to Toronto route, which should attract Business passengers, will be also be on the 757. And there is nothing stopping them operating the 757 on DUB-JFK/BOS/ORD in the future.
DUB-YYZ is purely low cost market at the minute, Air Canada Rouge taking it over and when it was mainline only 1 or of the 7 weekly flights had business class. Air Transit more less loco to. Going for 5 crew on this just would not be an option if they want to make profit. To much risk involved carriers can't afford risks now days.