The Health Service Executive has said that a radiologist at Cavan General Hospital discovered the radiology imaging glitch in the HSE system.
On 26 July, the radiologist came across the data error in which the '<' symbol, that was used in delivering information about a diagnostic procedure, was not visible when the scan was archived in the system.
The problem was immediately reported to the HSE and a fix devised within three days.
Over six years, 21,131 patients were affected by the flaw.
The HSE has been reviewing cases and while the size of the problem continues to be investigated, around 15,000 patients may need to be recalled.
All but two hospitals in Ireland use the National Integrated Medical Imaging System.
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HSE Director General Tony O'Brien has said the risk to patients is deemed low, as most clinicians would not solely rely on reports emanating from the system.
Yesterday the HSE said the recall of patients would have an impact on waiting lists and it would also be costly.
The HSE said the company that installed the technology had known about the glitch in January 2016.
Many hospitals around the world use the system, including 54 hospitals in the US.