The Irish carrier often offers inexpensive business class hops across the Pond, connecting to single cabin service in Europe. It has a codeshare partnership with American’s partner JetBlue, so they’ve sold tickets for onward connections at JFK on JetBlue. It was convenient, then, to have Aer Lingus in JFK terminal 5 co-located with JetBlue. With U.S. immigration pre-clearance in Ireland you get off the flight in New York as though it were domestic, and don’t even need to clear security to connect.
That’s going away, since starting April 27 Aer Lingus will be moving to JFK’s terminal 7 which means changing terminals for connections to JetBlue and therefore also re-clearing security.