Ryanair has condemned Fórsa for calling a fifth day of strike action on Friday 10 August, which will coincide with stoppages by colleagues in Sweden and Belgium.
The airline's pilots in Germany and Holland have also overwhelmingly backed strike action, but have not yet confirmed any dates for stoppages.
In a tweet, the company said it deeply regretted Fórsa's "unexplained" notice of a fifth strike by just a quarter of its Irish pilots, adding that it would "needlessly" disrupt another 20 flights and 3,500 customers during the peak travel period.
Ryanair condemns FORSA’s 5th unnecessary strike instead of accepting Ryanair’s invitation to meet on Tues 7 Aug: https://t.co/KJbUbgNLhZ— Ryanair (@Ryanair) August 2, 2018
Ryanair's Chief Marketing Officer Kenny Jacobs described the fifth Fórsa stoppage as irresponsible, unwarranted, disruptive and damaging to its business.
It also accused Fórsa of rejecting Ryanair's repeated offers to meet this Tuesday in a bid to resolve the dispute - adding that a meeting would now be "pointless".
Ryanair had issued the invitation to talks subject to Fórsa not calling any further strikes ahead of that meeting.
The airline says that rather than accept the invitation to meet, Fórsa had called a fifth strike for Friday week - proving that the union had no interest in meeting the company.
Ryanair claims this is because the process is being driven by a "...tiny handful of Aer Lingus pilots who are now working with other pilot unions across Europe to create a day of action on Friday 10 August next"
The airline called on Fórsa to remove any Aer Lingus pilots "outside the room" from the process, adding that they had no interest in meeting or resolving the row, but merely wanted to damage Ryanair for the benefit of Aer Lingus.
It claimed a handful of Aer Lingus pilots were working behind the scenes with pilot unions in Sweden, the Netherlands, Belgium and German to further disrupt Ryanair's business and customers.
Ryanair reiterated that it had agreed nine of the union's 11 demands for a seniority-based system for allocation of transfers, annual leave and promotions - though Fórsa has claimed that the airline is exaggerating the common ground between them.
The airline apologised to customers, and will be working to re-accommodate them over the coming days.
In a statement, the Irish pilots' union Fórsa said Ryanair's escalation of the dispute last Wednesday - when it threatened to sack 100 pilots and 200 cabin crew, or transfer them to Poland - had led to a predictable hardening of resolve among its staff.
The union says that for over a month, it has warned that industrial action would continue until there is "substantial movement on the pilots' reasonable demands" regarding an agreement on a fair approach to the allocation of base transfers, annual leave and promotions.
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Fórsa claimed that in the 19 days since the first one-day strike took place, Ryanair management had agreed to just two hours of talks.
Ryanair had invited the union to talks this Tuesday, 7 August, conditional on no further strikes being announced.
However, Fórsa says there cannot be a resolution if management persists with that precondition to talks, which it describes as "virtually unprecedented in similar situations across Irish industrial relations".
Fórsa reiterated that it remains available for talks, and is prepared to explore the option of third-party facilitation - which to date has been ruled out by management.