IAG's role is to a) decide where best to invest in growth and b) maximise cost and revenue synergies between subsidiary airlines.
If EI joins the transatlantic joint-venture, BA and other members of the joint-venture have to take a "metal neutral" approach not favour one carrier over the other for revenue purposes.
If you look at IAG at the moment, the most significant growth is coming from Iberia and Vueling. Iberia has just announced three new long-haul routes and a vast number of new/reinstated short-haul routes.
Many of Iberia's new short-haul routes are actually from the UK: London Gatwick, Manchester and Edinburgh to Madrid, and London Heathrow - Tenerife and Gran Canaria.
Meanwhile BA has been really getting in the neck of over short-haul profitability.
So, provided IAG can grow EI profitably, IAG will expand it, both in Ireland and the UK.