The budget airline has so far canceled 400 flights affecting over 70,000 passengers across Europe. They would have the option of a refund, rebooking on the next flight or rerouting.
The airline says 85 per cent of its Friday flights would operate and that affected customers would get email or text messages later on Wednesday.
Yesterday, at the eleventh hour, they were joined by Dutch pilots after an unsuccessful attempt by the airline to obtain a court order blocking the strike action.
Which? travel editor Rory Boland said the dispute resolution service used by Ryanair had already confirmed it would uphold a previous European ruling that crew strikes are not usually considered "extraordinary circumstances".
The Dutch pilot union, VNV, had earlier this week described today's strike as a "wake-up call" for Ryanair.
"Ryanair is the only multinational in Belgium that doesn't respect the Belgian law and that's not normal", said Didier Lebbe, a representative of union ACV-CSC, whose demands include securing its pilots pay when they are on stand-by.
But Ryanair, in a statement said "there will be no cancellations (of flights to and from the Netherlands) as a result of the unnecessary strike action by the Dutch pilot union". Irish pilots recently staged four one-day walkouts, while cabin crew in Spain, Belgium, Italy and Portugal went on strike on July 25 and 26.
Ryanair is facing a rising tide of protests from unions frustrated at the slow progress being made in negotiations over collective labour agreements.
"If this isn't available on the same or next day then we will accommodate you to your end destination on airlines with whom we have a reciprocal agreement".
Other concerns are the status of contractors whose working conditions are not as good as those enjoyed by permanently employed staff and employee contracts based on Irish legislation.
He added that Ryanair had already offered a 20% pay increase this year, and that 80% of its pilots in Germany were now on permanent contracts.
Among other issues, they are also seeking changes to Ryanair's practice of moving staff to different bases without much notice, and a reduction in hours.
But its combative chief executive Michael O'Leary has also warned the airline may shift jobs and planes to more profitable areas if the turmoil continues.
"Today our members are on strike to demand their rights".