Aer Lingus has not disputed claims that employees at its new Belfast hub will not be paid the same rates as staff south of the border.
Aer Lingus will offer "competitive local market rates" in NI
But a spokesman said it will offer "competitive local market rates".
He added: "Aer Lingus is currently involved in a detailed process to address unsustainable costs in its existing operations."
It has announced it is to create more than 100 jobs and launch new flights at Belfast International Airport.
Michael Landers of the Impact trade union was one of those raising concerns over pay difference between Belfast and Aer Lingus employees elsewhere on the island of Ireland.
He said: "We want to see the people who'll be working for Aer Lingus in Belfast paid as much and enjoying the same conditions of service and the same access to pension schemes as people in Aer Lingus enjoy at the moment."
Aer Lingus's Belfast venture means that flights between Shannon and Heathrow will stop with at least 40 resultant job losses anticipated.