The three initial patient cases, which prompted a major review of scans at University Hospital Kerry, were delays in cancer diagnosis.
The three cases were identified in July and August this year.
Speaking on RTÉ's Morning Ireland, the hospital's Clinical Director, Dr Claire O'Brien, said the issue which led to the review was not about resources at the radiology unit.
SIPTU Health Division Organiser Paul Bell has said the X-ray department at the hospital is a very unhappy place to work - writes rte.ie.
Speaking on the same programme, he said that radiographers raised issues about the department, and the management of it, in November 2016.
Mr Bell said a report, published in December 2016, identified a deficit in staff levels.
He called for an independent review of the unit and a report addressing staffing levels to be published.
He said: "The concerns of our members are being ignored, even though there's now an independent report that confirms a number of these concerns are bona fide and need to be addressed.
"Management have chosen, for the last 12 months, to ignore the fact that they need to engage. This requires urgent action."
A helpline to assist patients concerned about thereview will open from 9am.
The hospital is examining over 46,000 X-rays, ultrasound images and CAT scans .
During the summer, concerns regarding some scans at University Hospital Kerry emerged after complaints were made by a number of GPs and consultants.
Itled to a mass review, starting at the end of October, into over 46,000 images, involving over 26,700 patients.
The hospital still has to review over 28,000 images and hopes to complete this within ten weeks.
It says that, so far, a small number of serious delayed diagnoses have been identified and the patients have been contacted.
It says no critical failures have been identified to date from 18,000 images reviewed.
The hospital helpline will open from 9am to 5pm today, operating Monday to Friday.
The number is 1800-742-900.
Read more news of Dublin on our site.