My understanding is Aer Lingus are looking to use the the B757 in the following ways with Shannon as an example:
SNN SUMMER
SNN-JFK M-W-F-S A333
SNN-BOS -T-T-S- A333
SNN WINTER
SNN-JFK M-W-F-S B757
SNN-BOS -T-T-S- B757
This allows them to lease the A333 out during the Winter or conduct heavy maintenance etc.