SIPTU has warned that its planned strike by around 7,000 health and social care workers in so-called 'Section 39' bodies could escalate.
Several hundred more workers in ten further agencies are to be balloted for industrial action in their pay dispute.
Staff at nine such bodies have already voted overwhelmingly for a one day strike on 14 February over the failure of their employers to restore their pay in line with restoration for public servants.
The employers acknowledge that in many Section 39 bodies, pay was cut in line with public service austerity measures, even though Section 39 workers do not count as public servants.
The employers in question argue that their austerity block funding cuts from the Department of Health and the Health Service Executive have not been restored.
For their part, the Department of Health and the HSE say that they have sought additional funding for the pay restoration, but the Department of Public Expenditure and Reform has not given them increased funding to permit increases.
The Government contends that up to 2,224 agencies of varying sizes could be affected by any move to restore pay, at a significant cost to the Exchequer.
However, SIPTU Health Division Organiser Paul Bell insisted that only 66 Section 39 bodies have an established pay link to the public service.
He confirmed that while next week's stoppage is for one day, there will be a further two-day strike the following week if the dispute is not resolved.
Mr Bell accused the Department of Public Expenditure and Reform of "obstructing" a resolution to the dispute that concerned the union's "legitimate request for pay justice".
He criticised the fact that the department had refused to attend an Oireachtas Health Committee on the issue last week.
SIPTU says members in the following organisations will participate in the strike action on Wednesday 14 February and attend a demonstration outside the Department of Health in Dublin on the same day.
- Rehab Ireland
- The Cheshire Foundation
- Western Care in Co Mayo
- St Joseph's Foundation in Co Cork
- SOS Kilkenny
- Ability West
- Wicklow Community and Family Services
- SOS Workshops
- Marymount University Hospital and Hospice, Curraheen, Co Cork
The latest ballots will take place in Section 39 agencies in Wexford, Kerry, Westmeath, Sligo and Leitrim.