Ryanair faces a 24-hour strike by pilots based in Ireland on July 12 to coincide with the start of traditional summer holidays in Northern Ireland.
The industrial action is likely to affect thousands of holidaymakers who travel to Dublin to take flights at that time of year.
Although only pilots based in the Republic of Ireland voted to strike, the union warned it could have knock-on effects for flights elsewhere.
The dispute is over demands for a seniority agreement covering transfers between bases, promotion, annual leave and other issues governed by length of service.
The Irish Airline Pilots’ Association ballot of those directly employed by Ryanair resulted in overwhelming support – 99% – for action.
The 24-hour strike is due to begin at 1am on July 12 – the traditional holiday fortnight for people in Northern Ireland.
Ialpa, part of the Forsa trade union, said it would notify the airline of additional strike days in due course.
The union said its member pilots employed by the budget airline complain that there is no transparent system to address matters such as transfers between the airline’s bases, which can have a “devastating effect on family life”.
Ryanair described the strike as unnecessary, saying it gave Forsa proposals on union recognition, base transfers, a seniority list for Irish pilots and annual leave.
“Ryanair has invited Forsa to meet to discuss these proposals on 18 separate occasions but Forsa have failed to reply or take up any of these invitations to meet,” the airline told The Irish Times.
In a statement, Ryanair added: “Since Ireland accounts for less than 7% of Ryanair flights, we expect that 93% of our customers will be unaffected by any Irish pilot strike next Thursday.”
The union said it “remained available and willing to engage on the issues identified in the notice of strike action”.
In a separate development, the International Transport Workers Federation is planning a two-day summit for Ryanair cabin crews across Europe, to be held in Dublin.
It also warns of potential industrial action if Ryanair does not address its concerns.
German pilots union VC is also voting on possible strike action in a ballot that is due to run until the end of the month following a reported breakdown in talks over improved pay and conditions.