The airline has cancelled up to 600 flights over what it calls the cabin crew's "unjustified" action.
Ryanair had already said July 17 it would cancel 24 flights between the United Kingdom and Ireland on July 20 after the Irish Airline Pilots' Association (IALPA), a branch of union Forsa, gave notice of two more one-day strikes beginning at 0100 on July 20 and 0100 on July 24.
Ryanair is poised to cancel 16 further flights next Tuesday in its dispute with Irish pilots it employs directly.
Ryanair plans to cancel 24 flights between Ireland and Britain on Friday due to a strike by its Ireland-based pilots, the second of three planned for July, Europe's largest airline by passenger numbers said on Tuesday.
The Spanish government has said the cancellation of the flights is legal, but that it will ask the airline for detailed information regarding the flights so customers can take an informed decision in advance.More news: Trump thanks major pharma companies for not raising prices
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The letter, sent by Ryanair's chief people officer Eddie Wilson to Forsa national secretary Angela Kirk, who has been negotiating on behalf of the Irish-based pilots, referred to Wednesday's meeting with Forsa.
The airline said all affected customers had been contacted and would be re-accommodated or refunded.
"Ryanair cabin crew enjoy great pay -- up to 40,000 euros ($47,000) p.a".
A spokesperson said anyone affected had already been alerted by text message or by email, while also being offered an alternative.
The airline will consider issues including demands for local contracts by some staff, but the airline needs to maintain its "low-cost, high-efficiency" model, he said.