Ambulance personnel belonging to the Psychiatric Nurses Association are to escalate their industrial action over union representation rights with two 24-hour strikes.
The first stoppage will take place in the week beginning 27 May, with the second planned for the week beginning 3 June.
General Secretary Peter Hughes said that they would reveal the specific date "in due course".
The ambulance members will also hold a protest outside the Dáil on Wednesday 15 May.
They have already staged six 12-hour strikes, during which Defence Forces ambulance crews were deployed to plug gaps in service.
Mr Hughes warned that further strikes could not be ruled out, adding that the issue was not going to disappear.
Up to 2010, most frontline ambulance personnel were represented by SIPTU, but in that year a breakaway group called the National Ambulance Service Representative Association was established and subsequently affiliated with the PNA.
It claims to represent 500 of the 1,800 frontline staff, but that figure is disputed by SIPTU.
However, the HSE has refused to recognise the PNA/NASRA as it argues that it already recognises a number of unions, and to accept another one would impair good industrial relations and lead to fragmentation.
The HSE has also ceased processing PNA/NASRA union subscriptions through the payroll system - a facility that it offers to other unions.
Mr Hughes noted that all contact with the HSE on ambulance issues, including contingency planning, has taken place through their legal team and he reiterated his call for direct engagement.
The HSE also released a statement saying: "The National Ambulance Service cannot comment at this time, as it has yet to receive notification from the PNA."
PNA nurses criticise recruitment embargo
Meanwhile, the PNA has criticised the Government's introduction of a three-month recruitment embargo at a time when there are serious recruitment and retention difficulties in mental health services.
Speaking at the PNA conference in Wexford, Mr Hughes said there were already 700 psychiatric nursing vacancies in the system, but that would rise to 900 when 200 additional nurses required for the opening of the new forensic hospital in Portrane (to replace the Central Mental Hospital) were factored in.
The situation could further deteriorate in light of planned retirements.
He said delegates at the conference had expressed extreme frustration about the delay in engagement with the employer side, particularly as the INMO situation has now been resolved.
Talks at the Workplace Relations Commission will get under way on Tuesday, but Mr Hughes cautioned that the union's ballot for strike action remains live.