More than 400,000 passengers have been hit and it expects payouts to cost £17million on top of a £4million loss.
On Monday, 120 Ryanair pilots in Dublin reportedly attended talks at which a ballot for industrial action was discussed.
Its pilots around Europe are also understood to be considering action.
This could mean strikes or simply crews calling in sick. Ryanair dismissed “misinformation put out by the Aer Lingus pilots’ union. There is no truth in it”.
But more than 700 pilots quit in the past financial year, the Irish Air Line Pilots’ Association records, including 140 to rival Norwegian Air.
The airline partly blames the cancellations on a requirement to switch work year from April to March to the calendar year.
Chief Michael O’Leary said: “It is my mess-up there… I apologise.”
He added: “We are not short of pilots, we have 4,200.”
But, he said: “We don’t have enough pilots in September and October to allocate leave.”