Changes imposed by the Irish Aviation Authority in line with European law means Ryanair must bring staff holidays in line with the calendar year from January, requiring it to allocate that leave before the end of the year.
As well as air traffic control strikes and weather disruption, the airline said the decision was taken to meet a requirement for holiday time for its crews following the introduction of a new roster structure as required by regulators.
Europe's largest airline by passenger numbers sent emails to the first affected passengers on Friday, giving them the choice of a refund or an alternative flight.
The airline is now allocating annual leave during a nine-month transition period - April to December 2017 - to move the airline's holiday year - now April to March - to a calendar year (Jan to Dec) from 1st January 2018 onwards.
"The rules say if the airline doesn't have a suitable alternative flight, you have to be booked on a rival airline". Most stressful days of our lives'.
"We are calling on Ryanair to publish a full list now of all flights they intend to cancel".
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She wrote: "She's gone to bed in tears, rang hotel, can't get refund too short notice, same with parking, Anne Frank house tickets etc etc all non-refundable".
'Cancellation notices for flights cancelled up to and including Wednesday 20 September have been sent to affected customers and we will continue to send regular updates and post flight information on our website.
"Ryanair apologises sincerely for the inconvenience caused to customers by these cancellations".
According to a statement by the budget airline this was partly due to "increased holiday allocations" for pilots and cabin crew.
By reducing its scheduled flying programme over the next six weeks by less than two per cent (of its over 2,500 daily flights), the airline will create additional standby aircraft which will help restore on-time performance to its 90 per cent average.