The airline expected the travel plans of 42,000 travelers to be hit by the action in Germany alone.
The row over pay and conditions for Pilots is deepening for the budget airline as it faces growing action and it shows no sign of abating.
It is the biggest walkout in the history of the Irish airline as around a sixth of the company's flights are cancelled on Friday.
Dutch pilots' union VNV also called for a strike on Friday, after Ryanair said it was going to court on Thursday to try to prevent the Dutch pilots from striking over the summer.
Among their demands, which include sick leave and pension schemes, the European unions want the contracts of Ryanair employees to be governed by the laws of the nation where they are based, not by Irish legislation.
We again call on the VC to remove the threat of an unjustified and unnecessary strike, to commit to providing reasonable notice of strike action and to accept our invitations to meet for meaningful negotiations on a CLA for our German pilots and minimise disruption to German customers.
Ryanair said: "If your flight is cancelled there are two options available to you to choose from".
However, it has been unable to quell rising protests over slow progress in negotiating collective labor agreements.
Ryanair planes have a capacity of 189, meaning more than 74,000 passengers could be affected.
Yet, the head of German union Vereinigung Cockpit (VC), Martin Locher, blamed Ryanair for the strike, saying that "it has ruled out any rise in spending [and] has given no indication to its margins of manoeuvre to find a solution".
The airline has been grappling with staff unrest since it recognised trade unions for the first time in December 2017, in a bid to ward off widespread strikes over the Christmas period.
More than 74,000 holidaymakers have faced disruption as 400 flights have been cancelled.
It has further angered unions by threatening to move jobs away from bases affected by the stoppages, and began carrying that out in Dublin where it cut its winter fleet by 20% and put over 300 employees on preliminary notice.