Disruption and service delays in all Royal College of Surgeons (RCSI) hospitals including Cavan and Monaghan facilities are expected today.
It comes due to the planned industrial action by over 2,100 medical scientists over a pay dispute.
In a statement the RCSI Group pointed out that lab sample analyses will be extremely limited as a result of the action.
As a result, many inpatient and day-case elective procedures and outpatient appointments have been deferred.
Members of the public are being asked not to attend Emergency Departments "unless absolutely necessary".
"Laboratory sample analyses will be extremely limited and this has led to the deferral of many inpatient and day-case elective procedures and outpatient appointments," a spokesperson added.
"Where an appointment or procedure has been deferred, the patient has been contacted.
"We ask that you do not attend the Emergency Departments unless absolutely necessary on this day."
Meanwhile, the industrial action will last for 12 hours and concludes at 8pm.