The Health Service Executive is warning that planned industrial action by psychiatric nurses is likely to have a significant impact on mental health services from tomorrow.
Members of the Psychiatric Nurses Association say they will not be providing overtime from 7am tomorrow in protest at what the union says is a "lack of meaningful progress" in ongoing talks with the HSE.
The two sides have been involved in discussions at the Workplace Relations Commission since February.
The talks have centred on ongoing recruitment and retention issues.
The union says that little progress has been made in "over five months of engagement" and that no agreement is imminent. It says the goodwill of its 6,000 members can no longer be relied upon.
The union says the situation has been exacerbated by some of the actions of the HSE, including what it says is an effective embargo on recruitment, and a failure to offer permanent posts to those graduating this year.
The PNA says there are 700 vacancies in mental health services nationally.
In a statement, the HSE said it was assessing the situation on a service-by-service basis, and warned it was likely that the "withdrawal of cooperation with overtime would have a significant impact on services".
It said contingency planning was needed to ensure patients who are most at risk would continue to be cared for.