It said the additional 400,000 cancellations, which come on top of 315,000 bookings cancelled earlier this month, will cost it up to €25m ($41m), will "eliminate" all risk of further flight cancellations.
It appears that Ryanair has made additional cancellations because pilots refused to give up a week of their holiday in return for £12,000 ($19,500).
Mass delays announced earlier this month were caused by management "messing up" pilots' rotas, it admitted.
A spokesman said: "This slower growth will provide stability to pilot rosters from November to March. We will not need pilots to give up one week of their well-earned annual leave from November onwards.
"Slower growth creates a large surplus of standby pilots so we can allocate all annual leave due in the 3 months to December, and more again in Q1 of 2018."
All affected passengers have received a €40 travel voucher which will allow them to book a flight on any Ryanair service between October and March 2018, it said.
Ryanair has also offered the vouchers to the 315,000 customers whose flights were previously cancelled over a six week period in September and October.
It said it expected the cost of the free flight vouchers issued to affected customers to cost it around €25m.
The airline's shareholders have also been warned to expect "lower yields" from their investment as a result of "slower growth" over the next two months.
Hannah Maundrell, editor in chief at Money.co.uk said: "It's so disappointing for Ryanair passengers whose flights have been cancelled, even with more notice this time it's still seriously frustrating. Ryanair are still in a mess here, but you shouldn't be left out of pocket.
"If your flight has been cancelled, ask for a refund. You should get your money back within 7 days or given an alternative flight. This should also apply to connecting flights you miss as a result, as long as they were booked together.
"Aside from this, generally if your flight is delayed by over 3 hours you can claim compensation for the inconvenience. Cancellation amounts vary from 250 euros to 600 euros depending on your flight.
"If your other travel plans are impacted, look to your travel insurance for cover. This is when having a decent policy can really come in handy."