Ryanair has said it would cancel 40-50 flights a day from its schedule of over 2,500 flights for the next six weeks to the end of October.
The airline said it had been operating a "record schedule" which had resulted in record traffic levels during the peak summer months of July and August.
But it now has a backlog of holidays for pilots and crew members which must be taken before the end of the year in order to switch to a calendar leave year - as required by regulators - from January 2018 onwards.
The airline also said it was taking the move in an effort to improve its system-wide punctuality which has fallen below 80% in the first two weeks of September.
It said this was due to air traffic control capacity delays and strikes, weather disruptions and the impact of those increased holiday allocations.
The airline said that by cutting its scheduled flying programme over the next six weeks by less than 2%, it will create additional standby aircraft which will help restore on-time performance to its 90% average.
"Ryanair apologises sincerely for the inconvenience caused to customers by these cancellations," the airline said in a statement.
"Flights are operating as scheduled unless an email confirming a cancellation has been received. We advise customers to check the email address used to make their booking," said a spokesperson for the airline.
"Cancellation notices for flights cancelled up to and including Wednesday 20 September have been sent to affected customers."
"We have operated a record schedule and traffic numbers during the peak summer months of July and August but must now allocate annual leave to pilots and cabin crew in September and October, while still running the bulk of our summer schedule," the airline's Robin Kiely said.
"By cancelling less than 2% of our flying programme over the next six weeks - until our winter schedule starts in early November - we can improve the operational resilience of our schedules and restore punctuality to our annualised target of 90%," he added.