Ryanair pilots who are members of the IALPA union are set to strike for a fourth day on Friday the 3rd of August.
There's also warnings that more strikes may follow as the row over seniority and base transfers continues.
The latest strike date falls on the Friday of the upcoming August Bank Holiday.
In a statement today, the Forsa trade union said they believed the decision to issue protective notice to 300 of its staff today was an attempt to put pressure on its employees.
"Ryanair’s unnecessary decision to issue protective notice to 300 of its staff today is reckless and unnecessary, and demonstrates 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.
"The move also belies recent Ryanair statements that industrial action by the Irish Airline Pilots’ Association has had no impact on the company’s operations.
"It is normal practice for airlines to reduce activity in the winter months. In light of this – and of Ryanair’s recent difficulty in recruiting and retaining enough pilots to fulfil its schedules – it remains unclear if todays’ provocative move heralds a significant change in normal practice.
"However, the union sees today’s announcement as a provocative gesture which is likely to harden pilots’ resolve.
"Last Friday, the union wrote to Ryanair management to say it remained available for negotiations. There has still been no response to this. Fórsa remains available for talks, and believes it is possible to reach an agreement on the issues in dispute through negotiations."