Aer Lingus have by far the best profit margin in the group, just shy of 30% vs BA at #2 who are in the mid teens IIRC, so giving new A330s to anyone other than EI would be madness, hence why they got 2 this year and two more next year.
EI did okay operating MAD-IAD for UA a couple of years back, crews were based in IAD, and loved it apparently.