Aren't Aer lingus the dominant carrier on the route? Not sure why you think it's the British carrier who will be forced to surrender slots. Given the huge competition with the carriers below :
LCY-DUB CFE / WX / BE
LGW/STN/LTN-DUB FR
in the market, none of 'em are likely to want to have a wee go out of LHR. There's not much of a case for making anyone give up slot pairs. In terms of making sure ORK/SNN have access to LHR, genuine question, is it really so difficult to get to DUB and fly from there direct with a US carrier or the ME3?
Very contentious I know, not trying to provoke an argument, keen to know.
Your right about the route however for BA to get the Government to agree they may offer to give up slots to the commission (if required) or give EI the same number of slots back to the company if they were taken out of EI paris. The possibility of EI losing slots will not go in their favor and they will have to make up for it somewhere.
As for LHR generally, very overrated saw recent stat that around 2/3 of traffic from ROI is point to point. As for traveling to fly ex DUB, it is very expensive to fly T/A from DUB compared to London however an argument could be made if capacity was boosted it would help on that front.
FR have said they would likely take on LHR if they were forced to give up slots.