Some 34 people have been recalled after a review of medical scans taken at University Hospital Kerry.
All of the 34 patients had been told by their doctors the scans had not revealed serious illness.
It is likely these patients will now have to undergo new scans.
About 21,850 of the scans, including X-rays, cat scans and ultrasounds, have been reviewed. It is expected the number of patients being recalled will rise on Friday as more scans are reviewed.
The South, South West Hospital Group, which operated University Hospital Kerry, said it is currently reviewing 46,000 patient files following issues raised with the results for a number of patients.
It is understood that all the scans relate to work carried out by one locum who worked in the radiology department at the hospital. The person is no longer working at the hospital.
The hospital said the reviews completed accounted for more than half the patients impacted.
A telephone helpline for patients or former patients who might have concerns has been set up.
The number is 1800 742 900, and the line is open from 9am to 5pm on working days.
