Ten prisoners are being treated for suspected overdoses at Portlaoise Prison.
A number of prisoners were taken to hospital by ambulance after the incidents while others received treatment at the prison.
The Irish Prison Service (IPS) said it was “working closely” with the HSE in response to a number of overdose presentations in custody.
A spokesman said: “This a live operational situation for the IPS and the HSE. Both agencies are working to provide the highest quality health service to those impacted.
“A number of people are currently receiving medical treatment both in prison and in hospital.”
The prison service said “extra vigilance” is being taken and added that extra naloxone kits, which are used in the treatment of overdoses, had been secured.
In a statement, the IPS said preventing drugs and other contraband from entering prisons is a high priority.
It said: “The Irish Prison Service has committed to continuing to invest in new technologies and measures to support our efforts to keep contraband out of prisons.
“Prison staff have increased the use of random and intelligence-led cell searches on a daily basis. Our Canine Unit carry out searches around the prisons, including a greater focus on searching deliveries into prisons.
“The Irish Prison Service continues to engage with An Garda Siochana with regard to preventing contraband entering our prison and this happens at both local and national level.”
It has also carried out an information campaign for prisoners about the dangers of drugs.
The Department of Justice declined to comment on the incidents.
It comes less than a month after the IPS issued an urgent drug alert to all prisons about a “nitazene-type substance” following a fatal overdose.
The HSE National Drug Treatment Centre Laboratory was involved in confirming the presence of the drug.
