The Prison Service has said that the person who gained access to a restricted area of Portlaoise Prison was an authorised visitor.
They added that he did not simply 'walk in off the street' and that he had been cleared to enter the visiting area.
The Prison Service also said that the Garda investigation began immediately after the incident which happened before Christmas.
The man had been visiting a prisoner on the E4 landing before he was stopped by a prison officer and turned back.
Two separate investigations are under way into the incident.
The Irish Prison Service earlier said that the man did not have access to areas where prisoners are accommodated, but a senior prison officer was temporarily suspended in the aftermath of the incident.
Speaking on RTÉ Radio, Fine Gael Justice Spokesperson Charlie Flanagan said prison security was an issue that required a review of the highest nature.
