The strike is over improved pay and conditions.
The airline says it is cancelling 400 of over 2,400 flights today due to a strike by pilots also in Ireland, Sweden, Belgium and the Netherlands at the height of the vacation travel season.
The airline said 85 % of its Friday flights would operate and that customers affected would be informed by text.
They would have the option of a refund, rebooking on the next flight or rerouting.
Officials said flights should return to normal on Saturday.
Ryanair built its low-cost business model without unions, but said last year it would recognise them.
Labour representatives are seeking collective bargaining agreements in the different countries.
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