If (and it would seem unlikely) IAG were to be able to acquire Aer lingus, and retain all those LHR slots, then I guess is they'd slim down the Dublin transatlantic operation, and use the newly acquired LHR slots to add yet more capacity to the London / USA routes. I can also see BA using EI as a feeder operation to their LHR hub mainly, and cutting down massively on direct DUB - mainland Europe destinations, treating Dublin as it does Manchester, Glasgow and Edinburgh.
This would not result in UK regional passengers trudging down to LHR to fly with BA, but surely to drive more regional PAX into the hands of KL / LH and of course Icelandair.
However, I really can't see the UK or EU competition authorities allowing IAG to get away with retaining all of the EI slots at LHR were any such takeover to go ahead and I can't see the Irish government being overjoyed with potentially losing their "national flag carrier" and perhaps direct services to their main (EU) trading markets.