Health services will be severely disrupted on Thursday.
10,000 support staff are set to strike in a row with the HSE over pay.
Talks aimed at resolving the dispute ended without agreement at the Workplace Relations Commission last night.
Siptu's Paul Bell says the strike is now a certainty but that they will look to minimise the impact on patients.
"To be very honest we don't know what affect it will have on services," he said.
"We do know that there will be some affect.
"We'll have conversations this morning and probably tomorrow about what type of contingency we'll be providing but it will be very limited.
"At the end of the day there's been ample notice given of this dispute."
