A patient died when their diagnosis was delayed at Tallaght Hospital because x-rays were not reviewed by a consultant radiologist.
More than 57,000 X-rays taken at the hospital in Dublin between 2005 and the end of 2009 were not reviewed by a consultant radiologist.
The controversy only relates to adult patients and does not involve paediatrics.
The hospital says that to its deep regret, two patients had a delayed diagnosis as a result. One died last summer and another is undergoing treatment for cancer.
The hospital said that it informed the family of the patient who died as a result of a delayed diagnosis this afternoon. It said it had also spoken to the other patient today.
Tallaght Hospital CEO designate Professor Kevin Conlon said he was 'stunned' to learn of the problem when he was appointed on 14 December last year and was only informed today of the case of the patient who died.
Prof Conlon said the X-rays would have been reviewed at the time by a non-radiologist, most likely another clinician.
However, Prof Conlon added: 'This is totally unacceptable and it arose from systemic and process failures.'
In December, extra staff were appointed to clear the radiology backlog and to date, 34,752 of the 57,921 X-rays have now been reviewed and reports finished.
The hospital hopes to have the 23,169 remaining X-rays reviewed by May. The X-rays were for various tests including cancer, heart checks and orthopaedics.
Prof Conlon said: 'We have appointed a new Head of the Department of Radiology and we are interviewing next week for a new Clinical Director in Diagnostics.'
The hospital says the HSE was informed of the problem in December.
The HSE has said that it will meet with Tallaght Hospital management tomorrow to look at how the review of x-rays can be concluded as quickly as possible.
Fine Gael Health Spokesperson Dr James Reilly has expressed his sympathy for two patients and said he wants to know when the Minister for Health was made aware of the problem.
He added: 'The sheer volume and period of time during which X-rays were not reviewed by a consultant radiologist is astounding.
'We must now receive assurance from HIQA that this appalling situation does not exist in any other hospital in this country.'
To contact the Tallaght X-ray Helpline, call 1800-283059 from 9am-5pm.