Willie Walsh has already ruled out any interest in Aer Lingus based on Ryanair's shareholding. Given that one of the preconditions mentioned for a possible merger was accepting the current share structure, I can't see it being IAG. And even if it were I would see it as a hostile takeover given a successful Dublin hub is not in BAs interest and they will likely seek to funnel traffic lost to Aer Lingus Regional back through LHR.
If I had to put money on it, I'd say it was FlyBe (despite their backdoor deal with Ryanair. If anything it showed their expression of interest in a merger.)