Passengers on AA flights from Canada to the U.S. clear U.S. customs and immigration at pre-clearance facilities at the Canadian airports. Your AA flight will then arrive at JFK as if it were a domestic flight.
If AA has an interline baggage agreement with Aer Lingus, you may be able to check your bags all the way through to DUB when you check in at YYZ. Be sure to ask the AA agent who checks your bags in.
Passengers on AA flights