Fórsa says Irish-based Ryanair pilots will stage a further one-day strike this month.
It'll take place at 1.00am on Friday August 10th.
It'll be the fifth one-day strike by directly-employed Irish-based Ryanair pilots since July 12th.
This ties in with notice of action by pilots in Sweden and Belgium on the same day.
It's thought pilots in Germany and the Netherlands could also down tools on that day.
In a statement, the trade union Fórsa says: "For over a month, the union has said that industrial action is likely to continue until there is substantial movement on the pilots' reasonable demands for an agreement on a fair and transparent approach to base transfers and related matters.
"In the 19 days since the first one-day strike took place, company management has agreed to just two hours of talks, despite Fórsa's repeated assurance that it is available for discussions at any time.
"The union has today told Ryanair that it remains available for talks."
It claims the airline's "escalation" of the dispute last Wednesday - when it issued protective notice to 300 of its staff, or offered a transfer to Poland - led to "a predictable hardening of resolve among its staff".
Ryanair today (Thurs August 2) asks FORSA to confirm they will meet us at 11am on August 7. https://t.co/ZAZMVweLpe
— Ryanair (@Ryanair) August 2, 2018
Ryanair says it "deeply regrets" what it calls the "unexplained notice" of a fifth strike.
The carrier says Fórsa "again rejected" offers to meet to resolve the dispute on Tuesday August 7th.
Ryanair says: "Last Monday (30 July) Ryanair invited Fórsa and its Pilot Committee to meet on Saturday 4th August (or early next week) to discuss Fórsa's 11 requirements as a way to resolve this dispute without further customer disruption.
"Ryanair renewed this invitation on Wednesday 1st August and offered to meet FORSA at Dublin Airport at 11.00am on Tuesday 7th August.
"Rather than accept these invitations to meet (which Fórsa have repeatedly called for), Fórsa have now called a fifth strike for Friday 10th August, which will needlessly disrupt another 3,500 Irish customers."
Ryanair says it'll notify affected passengers, to whom they "sincerely apologise", of 20 further flight cancellations for August 10th.