It is understood that nine people overdosed after drugs were dropped into the prison yard. Four of the prisoners required hospital treatment, with one remaining in intensive care at Midland Regional Hospital Portlaoise on Saturday evening.
Two prison officers were also assaulted by a prisoner during the incident.
Following the overdoses, prison staff have increased the use of random and intelligence-led cell searches, while additional measures are being taken to detect and prevent contraband from entering prisons. Canine units are also carrying out regular searches.
The IPS said it remains committed to preventing drugs and other contraband from entering prisons and is working closely with An Garda Síochána to share information and prevent the trafficking of illegal substances.
The Prison Service has appealed to prisoners, visitors, staff and members of the public who may have information about the trafficking of contraband to come forward. Information can be provided confidentially via the free telephone line on 1800 855 717 or by text message on 086 180 2449.
