The Psychiatric Nurses Association (PNA) says the impact of a strike by 500 ambulance workers today will be significant.
Paramedics, advanced paramedics and emergency medical technicians with the union are taking part in their second 24-hour strike from 2pm today.
The union has warned that more strikes will follow if the dispute continues.
The ambulance workers are calling for the HSE to recognise their right to be represented by the PNA, which established its ambulance branch NASRA in 2010.
However, the HSE claims the union does not hold negotiating rights for ambulance grades and is refusing to automatically deduct membership fees from workers’ pay.
PNA General Secretary Peter Hughes said members are "not prepared to surrender" the right to be a member of a union of their choice.
He said: "Having campaigned for 18 months for this fundamental right, the resolve and determination of these PNA members is as strong as ever.
"They are fully supported by the wider PNA membership and a growing number of trade unionists from the wider trade union movement."
Union spokesperson Tony Gregg added: "If we surrender a cornerstone principle of the trade union movement, such as 'our union is our choice, then that would have a contagion effect throughout the system... where we would surrender that right on everybody else's behalf."
Today's work stoppage will continue until 2pm tomorrow.