I suspect that the leaning of Aer Lingus is towards Boeing for long haul; it could, of course, be a tactical thing. Obviously, they don't want Airbus to think they have EI in the bag. However, if EI's growth goes to plan, the potential size of the fleet could be double what it is now. Both manufacturers will be anxious to be a partner in that growth and given Boeing's recent success (despite the temporary AC problem), they will be anxious to get an A330 customer on board.
I think it may well come down to who will be the most flexible in being willing to provide aircraft when EI needs them. Again, EI may be able to use the size of the potential deal to help wrestle a more flexible deal. There is supposed to be an EU/US round of talks in the near future and even if it doesn't result in a deal, it may clarify matters, at least to an extent which would allow firm plans to be made. Being able to make firm plans for (say) April 2006 is better than a possibility of something happening in September or October.