Gardaí have confirmed they are investigating claims of unauthorised covert surveillance at the Midlands Prison.
It is after a major report published this evening found prison staff carried out the practice in 2011 and 2012.
The inquiry was ordered by the Minister for Justice last year into a special group aimed at tackling weapons, drugs and phones getting into prisons.
Among the findings is that a tracking device was attached to the private car of a prison officer and that proper procedures to deal with prisoner deaths were not in place.
The report also found the Irish Prison Service paid €29,000 to two private security firms to carry out tracking, surveillance and CCTV outside normal rules.
"The report found that there was malpractice within the prison system," said Justice Minister Charlie Flanagan.
"In particular in the Midlands Prison in Portlaoise where covert surveillance was discovered between the years 2011 and 2013."
Mr Flanagan said the practices were "unacceptable" and that it is "important that we learn lessons from that."