Aer Lingus has said that it has now taken 1m bookings in the 17 months since it began operating from Aldergrove International Airport in Belfast.
The establishment of the base in Belfast caused controversy because it involved switching Heathrow connections from Shannon.
It is the second biggest carrier operating from Aldergrove behind low-cost airline Easyjet.
Aer Lingus points to the load factor on its Belfast to London flights, as 79% of its seats were occupied, which is up a quarter on figures from April last year.
The Belfast to Malaga, Lanzarote and Paris routes are also proving popular.
The airline recently announced it planned to reduce some of its services from Belfast and Dublin because of the economic downturn.
In Northern Ireland, Ryanair operates from the smaller George Best Airport, near Belfast city centre.
It claims that compared to Aer Lingus at Aldergrove over the past 17 months, its services attracted 50% more passengers.
Ryanair says it did so while using one Belfast-based aircraft, compared to the three planes located at Aldergrove by Aer Lingus.
