The carrier has issued protective notice to 300 employees in Dublin
Irish-based Ryanair pilots are to strike a fourth day on Friday 3rd August.
The members of the Irish Airline Pilots' Association (IALPA) said further strikes may follow.
Their union, Fórsa, said more action would follow later this month unless the airline's management "changed tack" and "negotiated in good faith" on the issue of base transfers and related matters.
In a statement, the union said it believed the airline's decision to issue protective notice to 300 of its staff on Wednesday was "an attempt to put pressure on its employees."
"It said the provocative act, which was likely to harden pilots' resolve, had escalated the dispute while demonstrating management's unwillingness or inability to negotiate with unions in good faith."
Ryanair earlier approved plans to cut its Dublin-based fleet from 30 to 24 aircraft for the winter season.
It said these reductions have been driven by the "rapid growth of Ryanair's Polish charter airline... allied to a down turn in forward bookings and airfares in Ireland partly as a result of recent rolling strikes by Irish pilots".
It said these strikes have had "a negative effect on (close-in) high fare bookings and forward air fares as consumer confidence in the reliability of our Irish flight schedules has been disturbed."
The carrier said its Polish airline, Ryanair Sun, will now offer over 10 aircraft to Polish tour operators - more than double the five aircraft offered in summer 2018.
"We expect few route closures from Dublin, although some routes may suffer frequency reductions", it said.
In the light of the Dublin base cuts, Ryanair has issued letters of 90 days protective notice to over 100 pilots and over 200 cabin crew employees.
It said their services "may not be required" from October 28th onwards, due to this reduction in the winter fleet.
Ryanair said it will now begin consultations on redundancy, which, if redundancies are necessary, will be determined by Ryanair's assessment of flight performance, productivity, attendances, and base transfer requests.
The airline said it will also be offering transfers to Poland, and possibly some other bases.
Ryanair's Chief Operating Officer Peter Bellew said: "We regret these base aircraft reductions at Dublin for winter 2018, but the board has decided to allocate more aircraft to those markets where we are enjoying strong growth (such as Poland), and this will result in some aircraft reductions and job cuts in country markets where business has weakened, or forward bookings are being damaged by rolling strikes by Irish pilots."
It comes as Ryanair has been forced to cancel 600 flights across Europe on Wednesday and Thursday due to strike action by cabin crew in Spain, Portugal and Belgium.
The stoppage will affect tens of thousands of holidaymakers.