Good question! My understanding is that - if I enter Heathrow from the AA flight from the US with *only* carry-on bags, then I still need to go thru immigration/security to transfer to my ORK flight in Terminal 2..... but that if I have checked bags, but flights are on separate itineraries, then I will need to go thru immigration/security, and *also* through customs to pick up my checked bag, get it processed, and then check it in for my ORK flight out in the 'general check in' area.

If the above is correct, then of course I'd rather stick with just carry-on bags on the AA flight from the US, but I'm unclear if I can get over to T2 with my non-EMEA-size carry-on, and then just deal with it as checked baggage once I'm at the Aer Lingus gate.