Breathalyser tests were overestimated by almost one million over a five year period.
The Sunday times says an internal Garda investigation shows breath test figures were made up, input incorrectly and contained administrative errors.
The report was conducted by an assistant Garda Commissioner, Michael Sullivan, and is expected to be published next month.
Authors of the report also say Gardaí were not informed by the Medical Bureau of Road Safety about the issue.
The MBRS say they had alerted Gardaí of the problem with breath test numbers in 2014, but declined to comment to the paper.