When you arrive in T5 with Aer Lingus, you WILL need to take any checked luggage off the carousel and pass through Customs with it, handing in the Blue/white form as you do so. Once past Customs you will notice lines for AA and United and other airlines. This is where you take ur checked baggage and hand it over. Thus you will not need to travel to T3 with all your baggage.

If you already have been issued your boarding card and ur bags are TAGGED through to SEA you simply drop your bag off.

If you dont have your onward boarding card you still go to the AA desk just outside customs where they will retag your bag for SEA and issue your boarding card.

Basically, you will need to go to the AA interline area just outside customs! What seat have u been allocated on your Aer Lingus flights?

BTW a J ticket is a Business class ticket.