Aer Lingus grades represented by SIPTU have rejected negotiated proposals for pay cuts and work practice reforms.
The package would have kept pay at 50% of pre-Covid rates (rather than the current more severe cut to 30% of pre-pandemic pay) and averted layoffs.
Members voted by 55% to 45% to reject the recovery plan proposed by the airline.
"This is a very difficult and challenging time for our members in the aviation industry," said SIPTU Divisional Organiser, Karan O'Loughlin.
"We will now meet with workplace representatives to discuss the outcome of the ballot and to find a way forward that can protect the pay and employment of our members at the airline."
Cabin crew represented by Forsa had previously rejected a similar package, though the deal they balloted on had already been withdrawn by Aer Lingus management because unions had missed a deadline for acceptance.
However pilots have accepted a separate set of proposals which included fixed dates for gradual restoration of pay.
