11 prisoners in total were taken to hospital following the incident earlier this week.
One prisoner remains in ICU this morning following the overdoses at Portlaoise Prison.
It's understood he's in a serious condition but is responding to treatment.
11 inmates in total had to be taken to hospital after a suspected synthetic drug was taken on Tuesday.
An investigation is now underway into what exactly the drug was and how it got into the jail.
Former Garda Commissioner Michael O'Sullivan says it's impossible to stop drugs getting into prisons.