In the correspondence, dated today and addressed to all Ryanair pilots, Mr O'Leary said he was writing to apologise personally to each of them for the disruptions experienced to rosters as a result of the rostering management failure in recent weeks.
The Ryanair chief executive told the pilots the airline will each year benchmark its pilot pay against its 737 competitors, particularly Jet2 and Norwegian.
"He will be a hard act to replace", Ryanair chief executive Michael O'Leary said in a statement.
And then, later on in in September, Ryanair announced thousands more flights would be cancelled, with 34 routes suspended into spring next year.
No response has yet been made public from Ryanair's pilots or unions who now represent a portion of them.
Mr O'Leary added that Ryanair was a "very secure employer in a very insecure industry" - a reference to the collapse of Monarch - and called the airline's pilots "are the best in the business".
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In his role, Mr Hickey was responsible for scheduling shifts for pilots.
As the furor unfolded last month, O' Leary was forced to offer pilots a 12,000 pound bonus to forfeit their annual leave to ensure he had adequate cockpit crew.
Numerous airline's 4,200 pilots had joined unions over the past two weeks over discontent with the disruptions caused by the rota changes.
The company was already coming under heavy fire for cancelling up to 50 flights a day in the middle of September before an extra 18,000 flights were axed at the end of the month. These moves affect more than 700,000 passengers.
The airline says these problems to blame for her own bad decision to force pilots to use vacation until the end of the calendar year, and not until the end of the financial year in March.
But passengers have complained about the short notice of the cancellations and the consumer group Which? said Ryanair's compensation information was "woefully short".