SIPTU, the union which represents half of the workforce at Aer Lingus, has said that it will "vigorously oppose" the airline's takeover, unless IAG provides commitments that it will not seek redundancies.
Divisional Organiser Owen Reidy has said that the union is not being unreasonable.
"We are being told that this deal is going to grow the airline, it is going to substantially increase job numbers, and we are often told that the workers have made the airline what it is today," said Mr Reidy.
"So surely a commitment can be given for a policy position that guarantees no compulsory redundancies and that we will not outsource."