The budget airline announced that it will only maintain up to 65% of its route network between November and March.
It will close its bases in Cork and Shannon, both in Ireland, and Toulouse in France during the five-month period.
Routes that do survive will be served with a lower frequency of flights than normal.
Ryanair said its winter capacity will be just 40% of what it was 12 months earlier, compared with the 60% it previously planned.
It described demand for flights as "heavily curtailed" to and from the UK, Ireland, Austria, Belgium, Portugal and much of central Europe.
This has caused forward bookings to weaken "slightly" for travel this month and "materially" for flights in November and December.
The firm said it expects to fill 70% of seats on its planes.
It estimates it will fly around 38 million passengers during the 12 months to the end of March 2021, compared with 149 million during the previous year.
Group chief executive Michael O'Leary said: "While we deeply regret these winter schedule cuts, they have been forced upon us by government mismanagement of EU air travel.
"Our focus continues to be on maintaining as large a schedule as we can sensibly operate to keep our aircraft, our pilots and our cabin crew current and employed while minimising job losses.
"It is inevitable, given the scale of these cutbacks, that we will be implementing more unpaid leave, and job-sharing this winter in those bases where we have agreed reduced working time and pay, but this is a better short-term outcome than mass job losses.
"There will regrettably be more redundancies at those small number of cabin crew bases where we have still not secured agreement on working time and pay cuts, which is the only alternative.
"We continue to actively manage our cost base to be prepared for the inevitable rebound and recovery of short-haul air travel in Europe once an effective Covid-19 vaccine is developed."
Reporting by PA