500 ambulance workers are on strike today.
It comes amid a dispute between staff who are part of a branch of the Psychiatric Nurses Association (PNA) and the HSE over union recognition.
The crew members are picketing outside ambulance bases in Dublin and other locations across the country between 7am and 5pm.
The National Ambulance Service (NAS) says it has detailed contingency plans in place.
That includes having a number of crews from the Department of Defence on standby, managers carrying out front-line duties, and confirmation that ambulance crews from other unions will be working as normal.
The HSE says it already recognises other unions (SIPTU, UNITE and FORSA) for ambulance workers.
In a statement, the HSE said: "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.
"Recognising break-away unions has a destabilising effect on good industrial relations."
It adds: "The National Ambulance Service will stand by the agreements that it has made with recognised unions and will not undermine those agreements by engaging with other associations or unions."
The PNA, meanwhile, insists that the HSE is refusing to allow its members be recognised by a union of their choice.
On the subject of today's 10-hour strike, Peter Hughes - general secretary of the PNA - explained: "We will cover emergencies - emergency cover will be provided. Non-emergency [cover] will not be provided."
However, the PNA has said it's not calling on other unions to join today's action.
