The A330 that operates 4 times weekly to JFK is brought in to hangar for maintenance on the other three days. Aircraft are mostly swapped around in JFK, ORD and BOS depending on what a/c is needed for weekly maintenance. eg. if a/c comes from BOS on ei132 there will sometimes be an a/c change in SNN to continue on up to DUB. The a/c that will continue up to DUB on these occasions is usually the a/c that arrived in from JFK the same morning or sometimes ORD.
Hope that was clear enough.