The Irish Bank Officials Association has said it will fight any attempt to impose compulsory redundancies at AIB.
The bank today confirmed plans to lay off 2,500 people, with half of the job cuts by the end of this year.
It also said that it will have to consider "other options" if it cannot achieve the redundancies voluntarily.
The finance workers union, the IBOA, will now begin negotiations with the bank on a redundancy package.
Spokesman Larry Broderick has said workers are angry and upset.
Mr Broderick said: "It has been a very difficult place for the last number of years, I mean AIB staff have had this over their heads for the last 12 months and today doesn't bring any further clarity.
"Indeed what it says to them is that 500 more of them will have job losses than 12 months ago, but the mood is very angry, it's very upset. They would see themselves as not being responsible for the mismanagement of the bank and ultimately making the major sacrifice of losing jobs."