Although the conventional wisdom certainly favours Airbus in this case, Boeing should not be ruled out. Remember, Aer Lingus is waiting for a decision on new US access before deciding on a new fleet. With current disputes over cabotage (which EI isn't interested in), such a decision could take a long time, in which case EI is dependant on a special Irish/US deal.
Things becoming clearer now? "We'll give you access if you make the right fleet decision . . ."
In addition to this, EI has to ask whether the 333 is really the right aircraft for it to grow on t/a routes. The 332 is fine (although they really need to look into their service product, generally), but it does need to look ahead. A deal for a new aircraft would allow EI to inccorporate a new service product (PTVs etc.) and it would have an aircraft (the 772) with growth potential, which the 333 doesn't have.
Boeing will be well aware of EI's growth ambitions and therefore, it will see the airline as a potential 7E7 customer, which is a good incentive for giving them a good deal. If it can support and facilitate the growth of a good customer, it means more orders. Does Airbus have the same incentive to facilitate this growth? It may well feel that with the 320 order in the bag, it has EI in its pocket . . .
This will be an interesting contest.