The other nine inmates are understood to be in a serious condition.
One person is in a critical condition following a suspected mass overdose incident at Portlaoise Prison.
10 inmates were taken to hospital after a white powder was discovered there yesterday.
The prisoners were taken to Midlands Regional Hospital Portlaoise after the drug was discovered.
Nine of them are described as being in a serious condition with another person believed to be in a critical state.
A quantity of white powder has been sent for analysis with a result expected some time this morning.
Three weeks ago the Irish Prison Service issued an urgent drug alert to all prisons due to an analysis conducted by the HSE National Drug Treatment Centre Laboratory which confirmed the presence of a nitazene-type drug in prisons.
Nitazine has been associated with previous overdoses, one of those proving fatal.
Officials are waiting on the results of the analysis test before confirming what the substance is in this case.
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