More than 500 ambulance personnel who are members of the Psychiatric Nurses Association have begun a 24-hour strike.
They are in pursuit of a demand for the right to be represented by the PNA as their union of choice.
It is the second nationwide 24-hour strike by the PNA ambulance branch, involving paramedics, advanced paramedics, and emergency medical technicians.
Most frontline ambulance personnel were represented by SIPTU, but in 2010 a breakaway group called the National Ambulance Service Representative Association (NASRA) was established and subsequently affiliated with the PNA.
It claims to represent 500 of the 1,800 frontline staff, but that figure is disputed by SIPTU.
The HSE has said the National Ambulance Service is committed to maintaining positive industrial relations with all staff.
The General Secretary of the PNA has said his members are determined to be allowed to be represented by the union of their choice, not the employer's choice.
Speaking on RTÉ's News At One, Peter Hughes said the matter has been referred to the WRC, and the HSE has refused 'on three occassions at least' to engage under the WRC.
He said he believes it is a fundamental right for employees to be able to join the union of their choice, and to be represented by that union.
"We are determined to follow this through. We're gathering a lot of public and political support, and we're getting a lot of support from grassroots trade unionists. So our members are determined to continue this."
Mr Hughes said his members will respond to all emergencies during the strike, and that the main area affected is the transfer of patients from hospital to other facilities.
He said members have left the picket line when needed, and that the public is not in any danger.
