Well I'm pretty sure that SNN isn't exactly a premium market. Dublin I'm sure has high demand for premium service, but I think most T/A flights into SNN don't need EY levels of service, it's mainly VFR traffic on that route
UA attracts a decent level of high yield business passengers on their SNN-EWR service, so with a good product, EI could do well. If their was no demand for Business class, EI would operate the 757s in an all economy configuration.
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.