Latest: A court in the Netherlands has confirmed that Dutch Ryanair pilots are allowed to strike tomorrow.
The airline had tried to prevent them from joining their colleagues in Ireland, Sweden, Germany and Belgium.
Ryanair has has been forced to cancel one in six flights tomorrow, affecting the travel plans of around 25,000 passengers.
The majority of passengers affected have either been offered another flight or a refund.
Bernard Harbor from the trade union FORSA denies it is a co-ordinated action.
"Tomorrow will be the fifth time that Irish-based pilots are out on strike so I don't think there is any question of co-ordination because there's been no other pilots on strike anywhere else in Europe up to now," said Mr Harbor.
"We represent pilots that are directly employed by the company and based here in Ireland and that's the limit of our activity in this."
Update for Friday August 10th: pic.twitter.com/g9qWugyRr0— Ryanair (@Ryanair) August 9, 2018
Ryanair will go to a Dutch court later to try to stop pilots in The Netherlands joining 24-hour strike action alongside colleagues in four other countries tomorrow.
The Dutch pilots announced last night they would walk off the job after their union said efforts to negotiate a collective labour agreement had failed.
Ryanair has now cancelled hundreds of flights with up to 25,000 passengers affected.
Employment law professor at Maynooth University, Michael Doherty, says the strikes could cause problems for the company's long-term business:
Mr Doherty said: "The danger for Ryanair is, while it may contain the numbers of passengers affected on Friday, people are thinking about booking October, Christmas and New Year trips and the question is are they confident now Ryanair will get them out of this?
"And, to be honest, there isn't a great sign of any movement forward in terms of resolutions from the company side."