The Health Service Executive has said Army vehicles have been used today as part of its contingency plans for coping with today's strike by ambulance personnel.
Around 500 members of the ambulance personnel branch of the Psychiatric Nurses Association are picketing at locations across the country as part of the ten-hour work stoppage.
Martin Dunne, the HSE Director of the National Ambulance Service, said the service has responded to 259 calls so far today.
Mr Dunne said military vehicles had responded to five calls.
He said no patients had been left waiting longer than usual as a result of today's action.
Ambulance personnel belonging to the PNA began strike action at 7am in their dispute over union recognition.
Two further consecutive days of strike action are planned for 28 February and 1 March.
Earlier, the General Secretary of the Psychiatric Nursing Association has said it is "inexplicable" that the HSE would oppose the wishes of 500 ambulance personnel to be represented by the PNA.
Speaking on RTÉ's Morning Ireland, Peter Hughes said it is "totally unwarranted" that the HSE would oppose ambulance members of the PNA being allowed to join union of their choice.
Mr Hughes said it is not up to the HSE to decide what personnel from the health services the PNA can represent.
He said the 500 ambulance personnel want to join the PNA and the HSE "want to force them to join a union they don't want to join".
The HSE has said the ambulance personnel are well represented through the industrial relations machinery that exists.
It said that recognition of other unions or associations would "undermine the positive engagement that exists ... and it is not appropriate for the HSE to represent breakaway unions".
Mr Hughes strongly disputed this and said the PNA has operated for 40 years and is not a breakaway union.
Traditionally, around 1,800 paramedics were mainly represented by SIPTU, but in recent years up to 500 have broken away to join the National Ambulance Service Representative Association (NASRA), which is affiliated to the PNA.