INMO and SIPTU members employed at Caredoc facilities have announced plans to take strike action next month as part of a pay dispute.
The workers have threatened to withdraw their labour from 18 December.
Caredoc operates 14 treatment centres across seven counties.
It provides GP out-of-hours services, telephone triage and community intervention.
There are 150 INMO members and 120 SIPTU members employed at Caredoc.
The unions said they are seeking the implementation of a 2023 WRC pay agreement for Section 39 organisations, specifically an 8% increase due to members working in Caredoc.
Section 39 organisations are privately owned charities and agencies that are contracted by the State to provide healthcare services.
The unions said money paid by the Health Service Executive to Caredoc to fund a pay increase for staff has not been applied to members' salaries.
"Our members have no desire to withdraw their services in the middle of winter, but all other avenues have been exhausted," said INMO Industrial Relations Officer Gráinne Walsh.
"We will be meeting with workplace representatives to discuss next steps and ensure that all contingency and organisational arrangements are in place for work stoppages," Ms Walsh said.
SIPTU Health Organiser Ger McNally said that despite numerous attempts to engage with management, no resolution has been found.
"It is not acceptable for our members who work in a number of roles including call takers, drivers, receptionists, and administration staff to not have received their pay increases," Mr McNally said.
A spokesperson for Caredoc said that as the matter "is the subject of impending legal mediation, it would be inappropriate to make any comment at this time".
