The Director General of the Prison Service, Brian Purcell, has ordered an investigation into an incident last Wednesday when the wrong prisoner was released from Mountjoy.
35-year-old Mark Kenny was released from Mountjoy jail two days ago - by mistake.
Gardaí have yet to find him.
Kenny was not due for release until 2010, but he was let out instead of another Mark Kenny who was serving three months for a traffic offence and was due for release last Wednesday.
Prison officials say there were a number of remarkable similarities between the two Mark Kennys.
Both looked alike in their photographs, both signed their names in block capitals, both had similar appendix scars and both had tattoos with the letters E and R on their bodies.
However, the two men had different dates of birth and prison numbers.
The mistake was discovered just after 5pm Wednesday evening, some 20 minutes after the wrong Mark Kenny left the prison.
