Thousands of patients scheduled for surgery could have their appointments cancelled tomorrow, with up to 10,000 hospital support staff going on strike.
The workers - including porters, healthcare assistants and theatre staff - will stage a 24-hour walkout in a row over pay.
SIPTU members at 38 hospitals and healthcare facilities are set to take part in the action.
There are fears that some hospitals won't be able to provide regular meals and hot food to patients, with kitchen staff also going on strike.
The HSE says contingency plans are in place, but added that the strike will involve a "significant number of staff who make an essential contribution".
In a statement, the health service said: "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 remains committed to early resolution of the strike action given the number of staffing groups involved, and the direct impact on our patients in our acute and community services."
Talks aimed at resolving the strike took place earlier this week, but ended without agreement.