Ryanair's directly-employed pilots in Ireland will join colleagues in Sweden and Belgium on strike in an escalating day of action on August 10, the fifth one-day walkout in the airline's home market.
Ryanair has suffered strikes in some of its other main markets such as Spain and Portugal as it struggles to reach collective labour agreements across Europe, and expects to face action in Germany and the Netherlands as well on August 10.
The Irish airline, Europe's largest by passenger numbers, has responded by threatening to move jobs away from any bases affected by the stoppages, beginning with Dublin where it cut its winter fleet by 20% and put over 300 employees on preliminary notice.
"The airline's escalation of the dispute last Wednesday - when it threatened to sack 100 pilots and 200 cabin crew, or transfer them to Poland - led to a predictable hardening of resolve among its staff," Ireland's Forsa/IALPA trade union said in a statement.
The union said it was willing to explore the option of a third-party facilitating talks with Ryanair in seeking to bridge their differences.
Around a quarter of Ryanair's 350 pilots based in Ireland have taken part in the series of strikes so far, with the fourth scheduled for Friday.
Ryanair cancelled around 20 of Friday's 300 Irish flights as a result, mainly on busy routes to and from Ireland and Britain, and has said impacted passengers have either been put on another flight or refunded.
Ryanair, which operates from 86 bases in 37 countries and carried 130 million passengers last year, decided to recognise unions for the first time in its 32-year history last December to avert widespread strikes before Christmas.
Its shares fell further on Thursday and were 3.7% lower at 13.00 euros by 1430 GMT, the lowest mark since November 2016 and 8% below the level they sunk to in December when Ryanair shocked the markets by recognising unions.