Aer Lingus and US carrier JetBlue have formed a fully-fledged codeshare agreement.
The move comes five years after the pair signed a so-called interline partnership.
The codeshare agreement has been inked as Aer Lingus prepares to move its operations at New York's JFK airport to JetBlue's Terminal 5 in April. The Irish carrier currently uses Terminal 4 at the airport.
The existing interline agreement between the two carriers already enabled Aer Lingus passengers to make a single booking to fly to over 40 destinations in the US.
The new codeshare agreement differs because the two airlines will now market and sell each other's flights as if they were their own.
That means that JetBlue flights in the US will also carry an 'EI' flight number.
Aer Lingus is beefing up its transatlantic services. It's using an additional aircraft that had been in use by the now defunct service it operated in conjunction with United between Washington and Madrid.
From March, Aer Lingus will fly twice a day between Dublin and New York and Dublin to Boston.
It will also operate three flights a week from Shannon to New York as well as four to Boston.
Aer Lingus chief executive Christoph Mueller has said he remains keen to start a service to Canada.