The cases were detected as part of a review dating back to March 2013
Wexford General Hospital (WGH) has apologised, after 13 cancer cases went undetected.
In October 2014, WGH identified two cancer cases in patients who had recently undergone a colonoscopy - but were not diagnosed.
It conducted an immediate review of colonoscopies under the care of an individual consultant at the hospital.
All colonoscopy patients under the care of the consultant between March 2013 and November 2014 and their GPs were written to and received a follow-up phone call.
In total, 615 patients were recalled - 401 of whom were deemed necessary to have a repeat colonoscopy.
Over three-quarters of them attended a colonoscopy within one month of being contacted.
In total, 13 cancers were detected - including the two cases that prompted the recall, and the case of a deceased man who died before the review commenced.
The hospital says the consultant whose work this review concerns has been on leave since 2015.
WGH also began an immediate process of disclosure with patients and families concerned, and has apologised on behalf of the hospital for any failings.
The review - in line with the lookback policy of the Health Service Executive (HSE) - contains findings and recommendations which are being implemented, WGH says.
"The HSE is assured that services are operating to a high standard, but given the gravity and scale of this incident, the HSE is commissioning an external expert to review the Quality Assurance measures put in place and the overall management of the incident", it adds.
On behalf of WGH, consultant surgeon and clinical lead Ken Mealy said: "We deeply regret this incident. Since notification of the first cancer case, our priority at all times has been to conduct a thorough and immediate review.
"Our first action was to identify, recall and treat all patients who may have been affected.
"We have worked with BowelScreen to strengthen governance procedures around this incident and shared our learnings, with a view to ensuring that this never happens again."