"We are starting a series of discussions with our airports to determine which of Ryanair's underperforming or loss making bases should suffer these short term cuts and/or closures from November 2019," it said in a statement.
"This number could rise, or fall further, depending on when the Boeing 737 Max actually returns to flight services," the company said.
Boeing's 737 Max was grounded in March following two deadly crashes. Analysts had expected the planes to be back in the air by August, but Boeing does not expect to submit a software fix to US regulators until September.
That means the earliest Boeing and airlines can hope to have the plane flying again will be sometime close to the end of the year.
Ryanair (RYAAY) has ordered the 737 Max 200, a variant of the plane that will need to be separately certified by regulators. The carrier said it expects the model to be approved within two months of 737 returning to service.
The Irish carrier slashed its expected passenger growth rate for next summer from 7% to 3% as a result of the delay. That means it will fly 5 million fewer passengers than expected in the year to March 2021.
Ryanair said that it would work with Boeing and European regulators to catch up with deliveries in order to return to normal growth levels in summer 2021.
Boeing (BA) said in a statement that "we deeply regret the impact the 737 MAX grounding is having on Ryanair and their passengers."
"The regulatory authorities determine the process for certifying the MAX software and training updates and the timing for lifting the grounding order. We will not speculate on their schedule," a spokesperson added.
Carriers have been scrambling to adjust their operations. On Sunday, American Airlines (AAL) said it was canceling flights into early November because of the grounding. That amounts to roughly 115 flight cancellations per day.
Earlier this month, American became the first US airline to cancel a route due to the grounding, ditching a round-trip flight between its hub in Dallas, Texas and Oakland, California.
Ryanair said that it would consult with employees and unions after determining which airports would be affected by the cuts. It said the base closures "are directly caused" by 737 Max delays.
O'Leary said Ryanair would not change how it markets 737 Max flights to passengers once the plane is certified by regulators.
"We don't believe once the aircraft are back flying there will be any customer issues," he said.