Irish Nurses and Midwives Organisation and SIPTU members employed at Caredoc facilities have announced an escalation of planned strike action as part of a pay dispute.
The workers had previously threatened to withdraw their labour on 18 December.
They have now announced additional work stoppages over the Christmas period.
There will be 24-hour strikes on 23 and 27 December, a 48-hour strike from 29 December, and a 24-hour strike on 2 January.
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 at Caredoc.
The unions say they are seeking the implementation of a 2023 Workplace Relations Commission 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.
Caredoc said it will not be commenting on the matter, but in a letter to TDs and senators, seen by RTÉ News, the service said it supports the workers' campaign for pay increases.
The company said it had been forced to initiate legal action against the HSE to honour contractual obligations and to have all pay lost by staff over the past four years repaid to them.
The letter describes how a payment from the HSE of €640,000 was returned as it did not address the back money that amounts to in excess of €3.9m.
According to the letter, the planned strike action over Christmas would have a "devastating impact" for patient care and the hospital system.
"We are doing all in our power to prevent this industrial action from proceeding," the letter states.
"However, as part of our responsible planning, I must inform you that we are currently modelling (in a worst case scenario) for the loss of up to 10,000 hours of clinical consultations in the GP out-of-hours service and community intervention team services arising from this strike."
"The impact would be devastating for patient care and our hospital systems, with Emergency Departments and the National Ambulance Service placed under unprecedented pressure and extended patient waiting times an absolute certainty," Caredoc said.
