Ryanair staff pilots in Ireland, Germany, Sweden, the Netherlands, and Belgium were set to strike today for 24 hours, resulting in hundreds of the airline's flights being cancelled, including 20 to and from Ireland.
A Dutch court on Thursday evening rejected a case from Ryanair seeking to block pilots in the Netherlands from joining the strike, affecting about 22 flights.
Ryanair pilots are striking in five European countries, forcing the cancellation of a sixth of the firm's flights during the holiday season peak.
'The majority of customers have already been accommodated on another Ryanair flight.
Today's unprecedented simultaneous strike action by Ryanair pilots is the latest headache in a turbulent summer for Europe's second-largest airline.
One of the most severe was the 48-hour cabin crew strike on 25 and 26 July, which saw a total of 600 flights to and from Spain, Portugal, Italy and Belgium cancelled, ruining the travel plans of 100,000 passengers.
A spokesman said that despite the walkouts, 85% of Ryanair's scheduled flights, more than 2,000, would operate as normal.
Ryanair, which flies in 37 countries and carried 130 million passengers last year, averted widespread Christmas strikes last year by agreeing to recognise trade unions for the first time in its 33-year history.
But Ryanair pilots say they earn less than counterparts at other airlines such as Lufthansa.
Customers have been notified and a majority of those affected moved to another Ryanair flight, the company said.
The unions want the contracts of Ryanair employees to be governed by the laws of the nation where they are based, not by Irish legislation.
Kenny Jacobs, the chief marketing officer at Ryanair, said recently that the airline was "not prepared to concede to unreasonable demands that will compromise either our low fares or our highly efficient model".
But Ryanair insisted in a statement that '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'.
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.
Unions have strongly condemned what they see as Ryanair's attempts to play countries off against each other.
"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 carrier said.