IALPA pilots at Ryanair have announced a fourth one-day strike for Friday 3 August.
Pilots' union Fórsa said the company's decision to issue letters of protective notice was an attempt to put pressure on its employees.
It said the "reckless and unnecessary" move demonstrate's "management's unwillingness and/or inability to implement the airline's declared intention to agree working conditions with its staff by negotiating with their chosen trade union representatives".
Fórsa spokesperson Bernard Harbour said the decision will only strengthen pilots' resolve.
Mr Harbour said that the issue is resolvable through negotiations, but despite saying it was prepared to participate last December this protective notice is the first communication the union has received from the company.
This evening, Ryanair issued an ultimatum to Fórsa, saying that there will be no more negotiations while strikes by pilots continue.
In a statement the airline said it will meet the union in Ryanair offices, but not if any more strikes are called.
The statement added that plans are being finalised for up to 300 job cuts and that these numbers may be increased if further damage is caused to the business by the ongoing strikes.
The company claimed the union did not take up an offer for talks today and added that the pilot’s strike next week will only affect 20 out of 290 Irish flights.
Responding to the move, Mr Harbour said the dispute at Ryanair will not be resolved by issuing threats and that these issues can easily be resolved through negotiation.
He told RTÉ's Six One News that the airline's announcement today had escalated the dispute and was clearly an attempt to intimidate staff.
Mr Harbour said that if the company followed through on the contents of the letter issued this afternoon and did not meet the union, it would mean there will have been 17 days without any discussion on these issues and it would suggest that the airline is not serious about negotiation.
He said the union had suggested that a third party might assist both sides to meet.
However, he said what is certain is that these matters will not be settled unless both parties meet around a table.
Dublin-based fleet to be cut by 20% for the winter months
Ryanair issued letters of protective notice today to more than 300 staff as it announced plans to cut its Dublin-based fleet by 20% for the winter months.
The airline said it plans to cut its Dublin-based fleet of aircraft from 30 to around 24.
It said it has issued letters of protective notice to more than 100 pilots and 200 cabin crew employees, whose services may not be required from 28 October.
The airline said the moves come on the back of the rapid growth of its Polish charter airline, Ryanair Sun, along with a downturn in bookings and airfares in Ireland.
It said this was "partly as a result of recent rolling strikes by Irish pilots, which has had a negative effect on high fare bookings and forward air fares as consumer confidence in the reliability of our Irish flight schedules has been disturbed".
Ryanair said it will be offering transfers to Poland and possibly some other bases to the Dublin-based pilots and cabin crew employees for winter 2018 in order to minimise any redundancies.
It said it expected few route closures from Dublin as a result of reducing the fleet to 24 from 30, but that some routes may suffer frequency reductions as it moves the aircraft to its Ryanair Sun in Poland.
Ryanair's Chief Operating Officer Peter Bellew earlier said the airline regretted the base aircraft reductions at Dublin for winter 2018.
"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," he added.
Mr Bellow said that if the airline's reputation for reliability or forward bookings was affected, then base and potential job cuts such as these at Dublin were a deeply regretted consequence.
The airline said it has invited Fórsa and its Pilots Committee to a meeting this afternoon to brief them on the planned base cuts and job losses.
IALPA pilots at Ryanair are in dispute over over base transfers, seniority and leave.
The Irish Air Line Pilots' Association said there was no transparent system for the allocation of annual leave and promotion and is seeking an agreement to "provide our member pilots directly employed by Ryanair" with a fair mechanism to understand base transfers.
Transport Minister will not intervene, but wants an end to the dispute
Minister for Transport Shane Ross said he would not intervene in the dispute but wanted to see an end to it as soon as possible.
He said his worry was for everyone involved and "they should get back to work because passengers are being affected adversely."
Meanwhile, Ryanair crew in Italy, Spain, Portugal and Belgium are holding a 24-hour strike today.
On Monday, the airline reported a 20% plunge in profits to €319m for the three months to 30 June after being stung by lower fares, higher oil prices and pilot costs.
As well as the pilot strikes, Ryanair is also battling against disruption from air traffic control strikes in Europe and filed a complaint yesterday to the European Commission against France over the issue, alongside Aer Lingus owner IAG, EasyJet and Wizz Air.
Ryanair operates a fleet of more than 450 aircraft from 87 bases across Europe.