Around 500 ambulance personnel belonging to the Psychiatric Nurses Association are staging a 24-hour strike over union representation rights.
The dispute involves paramedics, advanced paramedics and emergency medical technicians.
The work stoppage will end at 7am tomorrow morning.
PNA members have already held six 10-hour strikes, during which Defence Forces ambulance crews were deployed to plug gaps in service.
Peter Hughes, General Secretary of the PNA said the 24-hour stoppage will pose "significant challenges", but the union has committed to responding to emergencies.
He said the dispute is about "ambulance members being allowed to join the union of their choice and be represented by the union of their choice".
Mr Hughes said that engaging with the PNA as a union which represents a large portion of the 1,200 frontline ambulance staff "shouldn't make any difference to the HSE".
The Health Service Executive said that a robust contingency plan has been devised and is being implemented around the country, and that senior management within the National Ambulance Service are monitoring the situation.
The HSE said that the volume of calls so far today is "as anticipated" and its ability to respond has been maintained.
"We are continuing to seek to ensure that service and care delivery are not compromised," a spokesperson added.
The PNA will announce further 24-hour strike dates in the coming weeks.
In a statement this afternoon, the HSE confirmed that: "The National Ambulance Service (NAS) is currently managing the situation, despite the challenges being experienced during today’s strike".
However, it added that: "Recognition of other associations or unions would undermine the positive engagement that exists and would impair good industrial relations in the National Ambulance Service.
"Recognising break-away unions has a destabilising effect on good industrial relations."
Dispute centres on formation of NASRA
Most frontline ambulance personnel were represented by SIPTU, but in 2010 a breakaway group called the National Ambulance Service Representative Association (NASRA) 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.
The HSE said the National Ambulance Service is committed to maintaining positive industrial relations with all staff.
In a statement today, the HSE said: "Ambulance personnel are well represented through agreed industrial relations processes. The National Ambulance Service recognises SIPTU, UNITE and Fórsa for staff in the service. In particular SIPTU is the recognised trade union for frontline staff".
However it added that "recognition of other associations or unions would undermine the positive engagement that exists and would impair good industrial relations in the National Ambulance Service.
"It is a well-established principle of public policy that fragmentation of union representation in the public sector is not in the interests either of the public or of workers. For that reason where grades of employee already have strong representation rights as is the case in the National Ambulance Service it is not appropriate for employers to recognise breakaway unions. Recognising breakaway unions has a destabilising effect on good industrial relations."
The statement said the principle of only engaging with recognised unions has been acknowledged by the Labour Court.
It said the National Ambulance Service will stand by agreements made with other unions and will not undermine them by engaging with other groups or organisations.
