The Psychiatric Nurses' Association has given the Health Service Executive three weeks' notice of commencement of industrial action over its recruitment embargo introduced last November.
The PNA has a membership of between 6,500 and 7,000 people.
Over the last three weeks, 96% of them voted in favour of industrial action over the HSE's recruitment embargo.
The union says its nurses are in an "impossible position" in delivering mental health services and that this industrial action is not about pay, but about patient care.
The HSE had refused to exempt mental health services from the recruitment embargo.
When the embargo was introduced last year, the PNA estimated around 300 vacancies of psychiatric nurses across the country.
According to its latest census of workers in the mental health sector, there are more than 725 psychiatric nurse vacancies across the country.
In a statement, the HSE said that despite the recruitment pause, the number of psychiatric nurses employed by the HSE increased in the first three months since the hiring freeze.
The HSE added it recently met the PNA and will continue to engage with it.
PNA General Secretary Peter Hughes said that mental health services are "at a critical point" and warned of possible closure of further services at community level resulting in significant risk in the community and increased admissions.
"There is also a risk factor in terms of clinical governance with the number of critical promotional posts that have not received derogation," he said.
Mr Hughes added that the number of vacancies is "seriously impacting on the delivery of care, the depletion of frontline services and the inability to develop services."
He added that yesterday, the Mental Health Commission published its annual report for 2023, which found a number of acute inpatient centre providers are struggling to meet standards in areas including staffing, care planning and risk management.
"The situation described by the Mental Health Commission has in fact deteriorated since November last," Mr Hughes said.
"The provision of basic psychiatric services throughout the country, including Child and Adolescent Mental Health Services (CAMHS) is reliant on both overtime and agency.
"This situation is unsustainable and there is real concern that the current situation will ultimately result in the closure of services.
"It is essential that the recruitment of psychiatric/mental health nurses is exempt from the recruitment freeze, as a matter of urgency."
Call for PNA to engage in talks
Minister for Health Stephen Donnelly has urged the PNA not to limit services to patients.
"I would encourage the Psychiatric Nurses Association to engage with the HSE and to use the industrial mechanisms of the State, and certainly not to be starting with actions that would curtail services for patients," he told the Dáil.
He paid tribute to the work of psychiatric nurses and said that he was "deadly serious about the expansion of mental health services".
Mr Donnelly was responding to Labour Party leader Ivana Bacik, who condemned "the scandalous recruitment embargo introduced last November".
At that time "there were approximately 300 vacancies for psychiatric nurses in this country. There's now a shocking 725 vacancies," she said.
The minister responded: "It is important to know, deputy, that in October, when the embargo was brought in, versus now, there are, in fact, 175 more psychiatric nurses in place. So even during the embargo, the psychiatric nursing workforce has increased."
