Mental health services face industrial disruption from Wednesday after pay talks involving the Psychiatric Nurses Association broke down without agreement.
A spokesperson for the PNA said the union had entered the talks in good faith, but felt insufficient effort had been made by the employer side to resolve issues.
As a result, the union will instruct its 6,000 members to reinstate their previously suspended industrial action by only working their contracted hours from Wednesday.
Both management and union sources have previously conceded that while an overtime ban falls short of a full strike, it still has the capacity to trigger disruption for patients in mental health services.
The spokesperson said that there would be exemptions from industrial action in certain areas, including disability and child and adolescent mental health services.
A spokeswoman for Minister for Health Simon Harris said the minister "would encourage the PNA to use the industrial relations machinery of the State, as the employer side is willing to do, in order to avoid the action going ahead on Wednesday.
"Disruption for patients should and can be avoided."
The PNA dispute centres on demands for additional pay and other incentives to recruit and retain nurses amid staff shortages that are affecting patient care and resulting in higher bills for more costly agency staff.
A similar dispute involving almost 40,000 nurses and midwives belonging to the Irish Nurses and Midwives Organisation was resolved earlier this year after three days of strike action, which virtually paralysed the health service.
The INMO resolution centred on a Labour Court recommendation that called for a new higher-paid Enhanced Nurse Practice grade, along with a broader entitlement to increased allowances, and a review of other relevant issues.
That deal was estimated to cost up to €50 million over 2019 and 2020 alone, subject to the delivery of as yet unspecified and unquantified cost-offsetting savings to be delivered through increased productivity and reforms.
Last week, in response to a Parliamentary Question from Fianna Fáil Health Spokesperson Stephen Donnelly, the Department of Public Expenditure and Reform confirmed that the estimated cost of the INMO deal in 2019 alone has now soared from €10-15 million to up to €57.7 million.
Many of the increases were due to come into effect from last March.
It said the estimated costs for 2020 were currently being assessed by the HSE and would be subject to the normal budgeting and planning process for the next financial year.
It said that a detailed implementation plan to give effect to this agreement is currently being finalised by the HSE, which will also set out key productivity measures which will be subject to independent verification.
However, neither the HSE, the Department of Public Expenditure and Reform nor the INMO has yet indicated what productivity measures are being sought, or the estimated value of such savings.
Without them, the cost of the INMO deal will spiral, throwing Government pay forecasts significantly off-target at a time when the Exchequer is under pressure from Brexit, as well as cost overruns on the National Children's Hospital and the National Broadband Plan.