Air traffic controllers have voted 99 per cent in favour of industrial action including strike.
A dozen flights were cancelled at Shannon Airport last night after controllers again refused to work overtime.
Management say the controllers are engaged in unofficial wildcat action but the controllers, who are members of Impact trade union, say they are not contractually obliged to work overtime and do so voluntarily.
They have taken the action due to what they say is chronic under-staffing. It takes some time to train controllers even if they are qualified and the company says it has hired around 35 people who will join the service on completion of their training late next year.
The vote was concluded on Friday and the details were released today. The union must serve two weeks notice of any industrial action it proposes to take but has not done so to allow time for internal discussion on what for industrial action should take.
The union said its members were working an average of 50.4 per week which was contrary to best practice and unsustainable.
"Until such times as the staffing levels are increased, an agreed and equitable system of standby and overtime needs to be put in place, but only as part of a series of interim measures," Impact said in a statement today.
The service at Dublin, Shannon and Cork airports had been understaffed since 2001 while air traffic volumes have increased by over a third, the union says.
Talks at the Labour Relations Commission are set for Tuesday week.