500 ambulance personnel around the country are half way through a ten-hour strike this lunchtime.
Members of the Psychiatric Nurses Association walked off the job this morning in a row with the HSE over union recognition.
The dispute centres on the HSE's refusal to recognise the PNA's ambulance personnel branch NASRA.
The HSE says engaging with yet another union will impair good industrial relations within the National Ambulance Service, while PNA members in the ambulance service are pursuing their right to be represented by the union of their choice.
Back-up plans are in place to deal with emergencies during today's strike and army personnel are on standby.
A number of PNA members are on strike today outside the Sacred Heart Hospital in Castlebar.