The Psychiatric Nurses' Association has begun targeted industrial action in Mayo, Galway and Roscommon in a dispute over staff shortages.
The action is a form of work-to-rule by nurses in the areas affected.
The PNA has accused the Health Service Executive of refusing to recruit all new graduate psychiatric nurses.
The action commenced in mental health services in areas where all new graduates have not been offered permanent contracts.
The PNA said the issue of the recruitment of up to 300 new graduate nurses is at the core of addressing what it described as chronic understaffing throughout the mental health services, which the union said is impacting daily on the delivery of services.
"While the PNA has secured nurse graduate positions in most areas of the country the issue has not been resolved in Mayo, Galway and Roscommon," said Peter Hughes, General Secretary of the PNA.
"This is hugely disappointing given the progress achieved in the rest of the country over the past week."
"This flies in the face of a HSE assurance given last February 2024 that all graduate psychiatric nurses would be recruited into our services," Mr Hughes said.
The PNA warned that the failure of the HSE to recruit graduates into the mental health services means they will be lost to the private sector and attracted to posts abroad.
"At a time when our mental health services are struggling to cope with 700 vacancies and are being maintained through a reliance on agency staff and overtime it is astonishing that the HSE can contemplate allowing this to happen," Mr Hughes said.
A spokesperson for the HSE said the action is due to affect the West and North West mental health area.
"We are continuing to work to resolve the issues involved," the spokesperson said.
