This would be a serious breach of trust on IAG's part. Aer Lingus considered the A330Neo and decided that it needed the A350s. For IAG to come in and reassign aircraft ordered by EI to another part of the group would seem like a retrograde step and inconsistent with IAG's commitment to grow DUB as a hub.
By the time the A350s are due for delivery, in 2018, EI will have been operating A330s for over 20 years; a capacity increase is needed if the airline is to build its hub effectively.
I think this question should be put to IAG and let them confirm or deny it. If it is confirmed, then perhaps the Irish govt should take legal action, on the basis that building the DUB hub was a key consideration in deciding to sell to IAG.