St Columcille's Hospital in Loughlinstown in Dublin is conducting a review of colonoscopies involving 65 patients.
It follows an examination of 160 colonoscopies performed by a doctor at the hospital between November 2017 and April this year.
As a result of the review, it recommended that 65 patients be called back for a repeat procedure.
A colonoscopy is an endoscopic examination of the bowel to look for abnormalities.
Up to yesterday, all but one of the patients had been contacted.
The review was sparked by concerns raised by one member of staff about the possibility that colonoscopies were not being completed by an individual doctor.
The Joint Group Clinical Director of the Ireland East Hospital Group said the hospital wanted to apologise to patients for any stress and inconvenience.
Kevin O'Malley said that the repeat procedure process was a "precautionary measure" and that the risk to patients was low.
This issue is not related to bowel screening but involves diagnostic colonoscopy, whereby patients were referred to the hospital by their GP because they had symptoms such as abdominal pain or bleeding.
Technology now allows hospitals to look back to see if a colonoscopy was fully completed by a doctor.
The hospital said that appointments were currently being scheduled and it expects the repeat procedure process to be completed by around the end of August.
A small number of patients have declined the repeat procedure, while others have asked to have it conducted later in the summer.
Referring GPs have been contacted.
The doctor involved worked at the hospital on a sessional basis and left around April.
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