IAG buying Aer Lingus would be 10x worse than Ryanair buying Aer Lingus.
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.
History paints a completely different picture. Just look at Buzz.
It might be worth recalling what happened at Vueling if we want to predict the near future.
IAG offered €7 a share and were unsuccessful. They then returned with an offer of €9.25 (a 32%) increase and succeeded.
I haven't read what price IAG had in mind with their recent offer to the EI board but even if it was as low as €2 per share, if they follow their Vueling strategy, they could return with an offer of at least €2.64.