IMHO, It's still all about the slots. I can not see any strategic value to IAG in the acquisition of EI other than the LHR slots (either in IAG getting them or preventing a competitor having them).
The devil will be in the detail. Unless there's explicit rules on the slot pairs/times/usage, suppose the following; the Irish government get a broad guarantee that the the number of EI slots used for routes to Ireland remains the same with IAG agreeing on the condition that the route remains profitable. In that case, couldn't IAG just move the EI flights to less revenue generative times, increase the costs base (say running a 321 rather than a 320), declare the route/slot pair unprofitable the reallocate to to a BA long haul?
Similarly, BA could just as easily drop their current DUB service and reallocate those slots, allowing the EI portion of IAG to run a monopoly on the routes as a BA feeder and jack the prices up.
Still, maybe within 7 years the politicians in the UK will get their act together and either approve a capacity increase at Heathrow or build a brand new bigger airport somewhere ... either of which would render the above 'conspiracy theories' moot.
JAS