They have been listed on www.ryanair.com and cover the period up to October 28.
He said that by focusing the cancellations on its busiest airports, where there are multiple flights each day, it would be better able to accommodate affected passengers on alternative flights.
"Compensation in the event of a cancellation also depends on..."
Mr O'Leary, the airline's chief executive, told a press conference: " Clearly there's a large reputational impact, for which again I apologise.
"By canceling less than 2 percent of our flying program over the next six weeks, we can improve the operational resilience of our schedules and restore punctuality to our annualized target of 90 percent", the airline said.
Ryanair could face up to €20 million in compensation claims after cancelling thousands of flights due to a shortage of pilots, it has warned.
"This is our mess-up".
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A Norwegian spokesperson said: "We can confirm that 140 pilots have joined us from Ryanair this year".
Throughout last Friday, the Ryanair Twitter feed was filled with complaints from passengers who had their flights cancelled, with one describing the lack of notice an "absolute disgrace" and another saying it was "shocking".
Mr O'Leary insisted the airline is " not short of pilots" as he explained the reason behind the cancellations.
More than 200 of the cancelled flights are either into or out of London Stansted, with a handful in Edinburgh, Manchester and Birmingham also affected.
The letter shows Ryanair pilots were only informed on 13 September of the staff shortage facing the company yet Mr Hickey outlined that it knew past year they may face a leave backlog.
Alex Neill, a managing director at Which? consumer service, said: "It's vital that passengers who have suffered the nightmare of Ryanair's cancelled flights are now given clear information about what they are entitled to".
Ryanair, Europe's biggest airline by passenger numbers, cancelled the flights because it had "messed up in the planning of pilot holidays".