The Health Service Executive (HSE) maintains that the recognition of other trade unions except Siptu, which represents frontline staff, would undermine work relations.
More than 500 ambulance personnel are staging a 24-hour strike in a row over union recognition.
The work stoppage ends at 7am tomorrow after commencing at the same time today.
The union represents paramedics, advanced paramedics and emergency medical technicians.
More 24-hour strikes will follow on dates yet to be announced if the HSE does not concede to the union’s demands.
Six 10-hour strikes have already been staged since January during which Defence Forces ambulance crews were deployed to plug gaps in service.
Peter Hughes, General Secretary who represents the National Ambulance Service Representative (NASRA) branch of the Psychiatric Nurses Association (PNA) re-iterated and that stoppages pose "significant challenges" but the union has committed to responding to emergencies.
The General Secretary 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".
“The irresponsible approach of the HSE in relying on its lawyers to negotiate contingency planning for a 24-hour strike involving front line emergency services is unprecedented.
“A year and a half after moving to deny ambulance personnel the right to be represented by the PNA, the HSE appears to persist in the view that this dispute will simply ‘go away’.
“After six days of strike action and an escalation now to 24-hour strike, our ambulance branch members could not be clearer that the dispute is not going away and their resolve and determination to be allowed join and be represented by the PNA (which has been the case since 2010) is as strong as ever.”
Frontline ambulance personnel were represented by SIPTU up until nine years ago, however in 2010 the NASRA breakaway group was established.
A HSE spokesperson explained 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.
In a statement, the HSE said that the volume of calls is "as anticipated" and its ability to respond has been maintained.
“Recognition of other associations or unions would undermine the positive engagement that exists and would impair good industrial relations in the National Ambulance Service (NAS).
"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."
In the statement, the HSE further added that the principle of only engaging with recognised unions has been acknowledged by the Labour Court.
“The National Ambulance Service will stand by agreements made with other unions and will not undermine them by engaging with other groups or organisations.”
A spokesperson added: "We are continuing to seek to ensure that service and care delivery are not compromised.”