IAG will efficetively shut down all transfer traffic from the Uk-Us via Dublin. The hub status that Dublin is hoping to achieve would be no longer possible and EI would probably pull out of Shannon and Cork.
Think that is most unlikely, what would be the point?
Surely the motivation for IAG to buy EI is:
(1) having seen (from the sidelines) LH asset strip BD's LHR slots after 2009, IAG would be reluctant to see another carrier/group do similar to EI at LHR and upset the current balance at "fortress Heathrow";
(2) if there's a profitable outfit with a thriving trans-Atlantic sector and a big presence at LHR up for sale, it would be stupid
not to buy it;
(3) it creates the ability to channel trans-Atlantic pax from airports not linked to LHR through DUB and take on KL;
(4) it creates the ability to offer trans-Atlantic pax from airports an additional option of using USA pre-clearance at DUB;
(5) to leave EI as an Irish "flag carrier" (it may be a regulatory requirement) because it's profitable, and because BA doesn't need LHR slots, there's no need to asset strip EI's LHR slots.