Update 7.45pm: Ryanair pilots in the Netherlands are to strike for 24-hours this Friday.
They will join their colleagues in Germany who earlier announced they would take part in the day of action, as well as pilots in Ireland, Sweden and Belgium.
The Dutch union says it has tried to negotiate a collective labour agreement without any result and says the strike should be a wake-up call for management.
The airline has cancelled nearly 400 flights due to Friday's joint industrial action - with more likely to be dropped.
Employment law expert Professor Michael Doherty from Maynooth University says Ryanair management are under increasing pressure: "What they'll say is something along the lines of it's going to be less than 25% of flights that will be cancelled on Friday but that is starting to ramp up now.
"It was 20 flights in Dublin now there will be 200 flights cancelled from Germany.
"So it is causing serious disruption."
German Customer Notice: pic.twitter.com/K3Pgt26qgi— Ryanair (@Ryanair) August 8, 2018
25,000 Ryanair customers are to be affected by cancellations on Friday as strikes take place in up to five countries across Europe.
Ryanair announced that it is cancelling 20 flights from Ireland on Friday ahead of strikes by Irish-based pilots.
This equates to just under 7% of the 300 scheduled flights, and the airline said that all 3,500 customers affected have been contacted.
Ryanair pilots in Germany also announced today that they will join strike action, with 96% of pilots voting for industrial action over improvements to pay and working conditions.
The strike by German-based pilots will affect all flights from Germany, Vereinigung Cockpit, the union representing pilots there, announced at a press conference this morning.
Friday's strike marks the fifth day of action by Ryanair Irish-based pilots, while pilots in Belgium and Sweden have already confirmed they are holding a 24-hour stoppage on the same day.
104 flights have been cancelled in and out of Belgium on Friday due to the strike.
The Dutch pilots union is also set to make a decision on whether they will take part, but they only have to give 12 hours notice to the airline for industrial action.
Meanwhile, there were delays to flights this morning due to Air Traffic Control staff shortages in France, Germany and Austria.
Ryanair said 73 of 440 first wave flights were effected, and apologised to customers for the delays.
ATC Update - August 8th: pic.twitter.com/fRwHPat1Df— Ryanair (@Ryanair) August 8, 2018