A union representing ambulance crews says a dispute with the HSE is exposing the problems faced by the service.
The Psychiatric Nurses Association says five ambulances had to stay off the roads in the south-east last night because the HSE didn't have stand-by staff.
The PNA's 500 members in the service are on an overtime ban, because of a union recognition dispute.
Sinead McGrath from the PNA says the HSE needs to engage with the union.
She said: "We want to be a positive influence on the service.
"We want to be able to represent our members certainly for their workforce rights but also for the betterment of the service.
"I'm a great believer in two heads are better than one so I'm not really sure why they're refusing to negotiate or enter into any discussions with us, especially now when we've reached a place where it is impacting on patient care."