Ryanair is facing widespread disruption across Europe with strike action in five countries, including Ireland.
Dublin-based pilots are walking off the job for a fifth day - joining their colleagues in Germany, Sweden, The Netherlands and Belgium.
Dutch pilots were given the go-ahead to strike after the airline had tried to prevent them from joining their colleagues.
However, members of the Dutch Airline Pilots Association (VNV union) will take part in a 24-hour work stoppage.
Almost 400 flights have been cancelled affecting tens of thousand of passengers.
.@Ryanair pilots picket at @DublinAirport as strikes in 4 other countries force the airline to cancel 400 flights @forsa_union_ie says its determimed to resolve the dispute but can't rule out further action pic.twitter.com/nyYtoLMnbD
— Paul Quinn (@PaulQuinnNews) August 10, 2018
In a statement, Ryanair said: "Despite the regrettable and unjustified strike action taking place in five of our 37 markets on Friday, over 2,000 Ryanair flights (85% of our schedule) will operate as normal tomorrow carrying almost 400,000 customers across Europe.
"Ryanair took every step to minimise the disruption and we notified our customers as early as possible advising them of their free move, refund or reroute options.
"The majority of customers affected have already been re-accommodated on another Ryanair flight.
"We want to again apologise to customers affected by this unnecessary disruption and we ask the striking unions to continue negotiations instead of calling anymore unjustified strikes".
Update for Friday August 10th: pic.twitter.com/g9qWugyRr0
— Ryanair (@Ryanair) August 9, 2018
Irish pilots also plan to picket Dublin Airport in their ongoing dispute over issues such as seniority and base transfers.
Bernard Harbor from the Fórsa trade union says both sides are still some way off reaching a deal.
"We believe that this is a problem that can be resolved through negotiations.
"But I have to say that the experience of pilots so far is that the company's not been prepared to really grapple with the issues - and that's why they feel that they've been forced into yet another day of strike action today".