More than 35,000 nurses are scheduled to strike in the Republic of Ireland on Wednesday concerning a dispute over pay and staff shortages.
The 24-hour strike is due to begin at 08:00 GMT.
Medical appointments are set to be disrupted for more than 25,000 patients.
The Health Service Executive (HSE) has put in place contingency plans to try and minimise cancellations.
The Labour Court said on Tuesday that it would not be intervening in the row following talks between the HSE and the Irish Nurses and Midwives Organisation (INMO).
The HSE has said that 13,000 hospital outpatient appointments and 2,000 surgeries will be cancelled.
A further 10,400 appointments at community care services will have to be postponed.
However, planned cancer surgery will go ahead as will disability services in residential units, while emergency departments will operate with reduced staff levels.
There are also strikes planned for 5 and 7 February, and then on the 12, 13 and 14 February.
The unions have argued that a pay rise is needed to recruit and retain staff, but the Irish government maintains that the 300m euros (£2.6m) it would cost is unaffordable.
Dr Emily O’Connor told Irish national broadcaster RTÉ that “about 3,500 to 4,000 patients” attend emergency departments at Irish hospitals every day.
“So there’s a lot of concerns that we won’t have enough nurses and that in particular we won’t have a nurse to triage patients as they come into the emergency department.
“That means we have concerns that we (doctors) won’t be as good at plucking out the sickest to be seen first.”
