Figures published on the Irish Prison Service website this week, showed the combined capacity across prisons in the Republic of Ireland has now hit 100%.
A female prison in Limerick has reached highs of 164% capacity, as numbers continue to grow.
In total, 4,416 people were in prison across the prison estate in Ireland at the beginning of the week, with only 4,411 beds available.
Today, prison numbers have reached 4,416 - five more than the number of beds available.
It was also reported that almost 50 people were sleeping on mattresses in prison in the last few weeks.
The Irish Penal Reform Trust has described the overcrowding as a "a watershed moment for Irish prisons" and that the prison population is "growing at a worrying rate."
The IPRT wants "strong action" from the Government, and for it to address the problem immediately.