Up to 30,000 hospital procedures will be cancelled due to a strike over the next two days.
2,100 medical scientists are picketing over pay and career development issues - similar to last Wednesday's action.
They're refusing to carry out routine lab services from 8am to 8pm today and tomorrow.
Tony Canavan, the chief executive of the Saolta hospital group, says the strike will badly affect patients.
"This is going to have a very significant across all the hospital services in the country, and across the Saolta group it will impact on patients who have already been cancelled for elective surgical procedures, patients who have been cancelled for endoscopy procedures, and also patients that have been cancelled for their outpatient appointments.
"The impact on patients is going to be very significant."
