The new base is likely to be found among the list of destinations currently served from Cork (although Glasgow isn't out of the question).
There is a possibility that it's just about streamlining operations at Heathrow, but if it isn't, your list of candidates is Birmingham, Manchester, Amsterdam, Paris, Munich, Malaga, Rome, Prague, Warsaw, Barcelona and Berlin.
You can rule out Amsterdam and Paris because they won't want to piss off AF/KLM. Munich doesn't really have enough of a volume to Ireland. Berlin and Rome are not much better.
That brings you down to Birmingham, Manchester, Malaga, Prague, Warsaw and Barcelona (with Glasgow still not ruled out).
The question is, whether they want to look at flying people from across Europe to the Mediterranean (Not likely IMO), providing services to Eastern European migrant workers (more plausible) or will play it safe with a service to somewhere the brand is relatively well known (most likely).
Logically, it would seem to strongly favour one of Birmingham, Glasgow or Manchester. Looking at competition and slot issues, Birmingham looks like a slight favourite.
Long-haul from the new base wouldn't start for a long while yet. EI don't have enough aircraft to fly all the desired long-haul routes from Dublin, never mind a new base.
Also, talk of long-haul destinations not in the current network is very wide of the mark.