The Health Service Executive (HSE) has warned of significant disruption to services ahead of a planned strike on Thursday.
Thousands of patients could have surgeries cancelled as up to 10,000 healthcare workers down tools.
Ahead of the 24-hour work stoppage, there is still no sign of a resolution between the HSE and hospital support staff.
Porters, cleaners, chefs, lab assistants and maternity care assistants are among the workers that will take part in the walk-out.
The dispute has arisen over pay, with SIPTU saying the staff in question have not received a pay rise recommended through a job evaluation scheme.
The HSE has said it "regrets that industrial action planned by SIPTU... will affect patient and client services."
The industrial action is planned to take place at 38 hospitals and healthcare facilities, and will involve a "significant number" of staff who make an "essential contribution" to the effective running of the health services.
"While every effort will be made to minimise impact on patients, industrial action involving these essential staff will have a significant impact on services."
The HSE added that it is continuing to engage on contingency planning with SIPTU at local hospital and healthcare facility level.
"This is to ensure minimum disruption to patient services, in so far as possible, and to ensure patient dignity and that essential daily care remains in place", it said.
Initial feedback suggests the services mostly impacted will include:
- Deferral of some elective inpatient procedures
- Significant cancellation of scope procedures
- Reduced outpatient services
- Reduced laboratory services for GPs
- Reduced catering services for both patients and staff
- Reduced operating theatre activity
However the services impacted will vary across the hospital sites.
Patients are being contacted by their local hospital or healthcare facility in the event that their scheduled procedure or service will be affected by the dispute.