Frequent traveller DUB and SNN to JFK and EWR. My experience: Aer Lingus, Delta flights to JFK from DUB and Continental's flt. 23 (9.00 departure) pre-clear immigration not customs (latter facility at DUB not operational yet.) Upon arrival in US, a passenger is triaged through immigration but must collect any bags, clear (usually speedy) customs, and re-check bags for ongoing connection. CO127 (10.50 departure) does NOT clear immigration, necessitating immigration queue upon arrival in US.

From SNN: Continental, Delta, and USAir (to Philadelphia) preclear both immigration and customs. Bags are transferred by carrier or collected at domestic baggage carousels (warn your driver or greeter!.) Aer Lingus are being punished for not using the overstaffed, over-thorough facility at SNN because they really cannot arrive domestically and then depart internationally--they use JFK's Terminal 4 which is international only. How punished? Departing flights no longer preclear immigration. Then again, they have severely curtailed SNN-JFK service. . .