A major area of attention on the part of IAG over the past couple of years has been to standardise deliveries of new aircraft across the group so aircraft can be easily moved between airlines to meet market & seasonal demand.
Just because a new aircraft is delivered to one IAG airline doesn't mean it rests with them in perpetuity.
I think IAG is far better placed than any Government to decide what new aircraft are best suited to Aer Lingus. Government interference didn't help with Iberia (which is now expanding again without any pressure from Government) and it won't help Aer Lingus.
As IAG gets bigger, I suspect we'll see a trend of BA, EI having some extra aircraft for the summer season and these aircraft being repainted and going elsewhere for the winter.