A number of Irish-based Ryanair pilots are to take pre-Christmas strike action following a dispute over winning independent representation for pilots in the company.
The one-day industrial action will take place on Wednesday December 20 and will mostly involve captains.
Trade union Impact said it would either disrupt flights or generate substantial costs for the airline.
Pilots from the Irish Airline Pilots' Association backed the industrial action overwhelmingly during secret ballots.
Although the number of employees involved in the strike is fewer than the total number of Irish-based Ryanair pilots, the action will have impact because planes cannot legally or safely fly without a captain, the union added.
Ryanair said some disruption may occur, and blamed the strike on a "small group of pilots" who are working their notice periods.
Impact official Ashley Connolly said Ryanair was the only Irish-based airline that refuses to recognise independent pilot representatives.
The action relates to 79 pilots directly employed by Ryanair, less than 28% of its Dublin workforce.
The budget airline suffered a turbulent period earlier this year when it cancelled flights between September and March after mismanaging pilots' annual leave, affecting around 700,000 passengers.