One of the those affected is understood to be in a critical condition.
A further two prisoners have been taken to hospital following a suspected drug overdose at Portlaoise Prison.
11 inmates have now been affected and taken to Midlands Regional Hospital Portlaoise - three of them are receiving intensive care.
It's understood six of affected inmates have returned to the prison.
A batch of drugs were found in a cell in the C wing, which will now be analysed.
General Secretary of the Prison Officers Association, Karl Dalton, says it was inevitable:
Crime Editor with the Irish Times, Conor Lally, says lab produced drugs are gaining in popularity here and are extremely dangerous:
The POA says overcrowding is contributing to the drug problem in Irish prisons.
The institution was at 107% capacity yesterday, with 16 more prisoners in custody than there were beds available.
General Secretary of the POA, Karl Dalton, says overcrowding makes their members' jobs more difficult:
In a statement to Midlands 103, the Prison Officers Association says the situation is 'deeply concerning.'
Gabriel Keaveney, Deputy General Secretary of the POA, says, despite the best efforts of its members on the ground, no significant action has been taken by the authorities to address the drugs issue in Irish prisons:
"What has regrettably now occurred in Portlaoise Prison was somewhat inevitable.
This is the second major incident involving illegal drugs in our prisons in three weeks.
In recent weeks we have had a serious attempt to burn down a Prison Officers home.
In recent times we have highlighted the issue of drones delivering drugs, mobile phones and weapons to prisons on an almost daily basis.
And we have consistently raised the overcrowding issue, which of course is central to all other issues which emerge in our prisons.
The Minister and the prison authorities need to see this incident in Portlaoise as an inevitable consequence of inaction over the years.
The chronic issues impacting on our prison system must be addressed or we will be confronted with significant crisis events in the very near future."
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