Aer Lingus have made enormous efforts so that in their new base they will not have to pay the the same salary as Irish staff, but that they will only have to pay the local market rates. In this respect, the prospect of a UK base doesn't add up.
Irish wages rates are now amongst the highest in Europe and are generally higher than those in the UK so they might not have to pay UK staff as much as Irish staff
Dublin, Cork, Shannon
Whatever about Dublin and Cork, I'd say its fair to say that EI have abandoned shorthaul at Shannon to Ryanair so I'd say any new routes there are highly unlikely
Riga
Vilnius
Warsaw
Krakow
Warsaw already has LOT, Wizzair, Centralwings and Norwegian based there, as well as numerous others flying there, I don't think theres really much potential there. As for Krakow, every second airline already flies there so there are few unserved routes. Riga or Vilnius are more probable, but Aer Lingus is stiill a fairly unknown brand there so I'd say they might struggle a bit in either of those cities.
Flying to their new base from Paris or Amsterdam would be a very risky strategy since KLM could decide to withdraw the Codeshare arrangement for AMS-Dublin/Cork, and both of these routes would struggle at current frequencies without it.