Ryanair pilots in Portugal are also set to strike next week.
The Portuguese union SPAC says the company's refusal to enter negotiations left them with no choice.
They will now join Dublin-based IALPA pilots in a 24-hour strike next Wednesday, December 20th.
In a statement published by Publico, the Portuguese union explained: "This action is related to other movements by Ryanair pilots in Ireland and Italy, and SPAC deeply regrets that potential disruption to passengers' travel plans may arise."
The group says that the action is being taken due to the airline's "continued refusal to negotiate with pilots in a fair and transparent manner".
Meanwhile, Ryanair pilots in Italy are due to hold a 4-hour stoppage on Friday, while German pilots have also threatened strike action.
Ryanair has not yet said how many flights will be cancelled, simply noting it will deal with any disruptions "if or when they arise".
In a statement yesterday about the action by Irish-based pilots, the company said the threatened action is by less than 28% of its Dublin pilots.
It added: "While some disruption may occur, Ryanair believes this will largely be confined to a small group of pilots who are working their notice and will shortly leave Ryanair, so they don’t care how much upset they cause colleagues or customers."