A review is being conducted of more than 46,000 x-rays, CAT scans and ultrasounds at University Hospital Kerry.
The South/South West Hospital Group says it is investigating patient safety concerns at the hospital.
Scans of around 26,000 people are involved, with two or three scans per person in some cases.
The review covers the period of March 2016 to July 2017 and as of last Friday, 18,000 files have been reviewed.
The hospital group says only 21 patients have been called back for "repeat imaging", and none of these required any further follow-up or treatment.
The review is expected to take up to 10 weeks and the hospital has set up a dedicated information line, 1800-742-900, which will operate from tomorrow between 9am to 5pm, Monday to Friday.