Ambulance personnel belonging to the Psychiatric Nurses Association have warned that they are prepared to escalate their strike action in a dispute over union representation rights.
The warning comes as the paramedics and emergency medical technicians hold their fifth 12-hour strike over the Health Service Executive's refusal to recognise the PNA's right to negotiate on behalf of members.
A further stoppage is scheduled for next Wednesday 10 April.
Up to 2010, SIPTU represented most frontline personnel in the National Ambulance Service - but since then, the PNA says up to 500 staff have left SIPTU to join its National Ambulance Service Representative Association (NASRA) branch.
However, the HSE insists that recognising another union for the ambulance grades would lead to fragmentation and impair good industrial relations.
It has also refused to deduct and remit union subscriptions for ambulance members to the PNA through the HSE payroll system as it does for other unions.
Speaking on the picket line at the Davitt Road ambulance depot in Dublin, PNA General Secretary Peter Hughes said the HSE was still refusing to engage through the Workplace Relations Commission, despite three requests.
He said that after next week's strike the union will review its strategy, and plan how to escalate its industrial action.
Mr Hughes insisted that the membership was growing, and they were getting support from other ambulance personnel.
Asked what support the PNA had received from other unions, he said there had been no contact with any unions affiliated to the Irish Congress of Trade Unions.
Asked whether it was disappointing that the PNA had not received any support from other unions, he said it was disappointing, but noted that his union had not requested support.
He said that as far as the PNA was concerned, this was a matter between the PNA and the HSE.
Asked about the possible options for escalation, including 24- hour stoppages or an all-out strike, Mr Hughes said they were not ruling anything out.
He said the PNA had a 49 year history of representing staff in the HSE - and members were determined to follow through on their campaign for the fundamental right to be represented by the union of their choice.
He called on Minister for Health Simon Harris to instruct the HSE to engage under the state mechanism of the WRC, adding that this was not a "big ask".
Meanwhile, the HSE has confirmed that the National Ambulance Service (NAS) is currently managing the NASRA strike situation.
An agreed contingency plan is being implemented with management within the NAS monitoring the situation.
The NAS received a total of 240 calls this morning, three of which were responded to by military ambulances.
While the air ambulance service remains available, there have been no calls requiring helicopter assistance so far today.
The HSE says the National Ambulance Service is committed to maintaining positive industrial relations with all staff and that ambulance personnel are well represented through an agreed industrial relations processes.
It says the National Ambulance Service recognises SIPTU, UNITE and FÓRSA for staff in the service.
It says recognition of other associations or unions would undermine the positive engagement that exists and that it would not be appropriate for employers to recognise break-away unions.