Ambulance workers are refusing to work overtime as they demand the HSE recognise their chosen union.
Up to 500 people are refusing to work additional shifts from today.
They are protesting over claims the HSE will not engage with the Psychiatric Nurses Association on their behalf.
This is phase two of industrial action in the dispute which has been ongoing since earlier this year.
The ambulance personnel branch of the PNA, NASRA, has consistently grown its membership since it formed in 2010.
General Secretary, Peter Hughes, says there have been issues their members wanted the union to represent them on but the dispute with the HSE has prevented this from happening.
"There have been a number of things, there have been changes to rosters without any consultation with our members," he said.
"That's coming in a number of areas, and all the options that should happen under consultation under all national agreements, however, our members have been denied that basic right and have had to issue grievances in relation to rosters being imposed without any consultation, that's one example," he said.