I must say that the third based 757 thing is probably wishful thinking and nothing more than that (the conclusion many have already probably arrived at). Aer Lingus would positively haemorrhage money by undertaking to compete in those markets, especially against an airline it's already codesharing with. If a fourth 757 came to the fleet, I'd put my money on it doing routes ex-DUB, it just makes sense as it's Aer Lingus' transatlantic hub.
As alluded to by others, more interest is in the YHZ proposals ex-DUB, but that'd be best discussed in the Aer Lingus thread.