The Irish Penal Reform Trust (IPRT) has expressed "grave concern" over figures which show that the number of beds across Irish prisons has reached capacity.
It said the number of people in prison in Ireland has exceeded the number of beds available.
The official figures, published on the Irish Prison Service website this week, showed the combined capacity across prisons in the Republic of Ireland has hit 100%.
One prison - Limerick (Female) Prison - has reached highs of 164% capacity, as numbers continue to grow.
In total 4,416 people were in prison in Ireland this week, with only 4,411 beds available.
The IPRT has pointed out that the number of people in Irish prisons was just 750 in 1970; 1,200 in 1980; 2,100 in 1990; and 2,948 in 2000.
It said as numbers continue to rise, news of almost 50 people sleeping on mattresses in prison in the last few weeks highlights the extent of the issue.
The IPRT is calling on the Minister for Justice to make immediate efforts to adequately resource the proposed actions in the recently published Review of Policy Options for Prison and Penal Reform.
It said this will reduce the number of people sent to prison and support a move away from responses to offending that cause unnecessary further harm to people and their families.
IPRT Acting Executive Director Molly Joyce said that while the Government has indicated that its committed to exploring the policy of "prison as a last resort" by providing alternatives to prison such as community-based sanctions where appropriate, she said the growth in numbers of people in prison shows that the policy is far from reality on the ground.
"This is a watershed moment for Irish prisons. We now have a prison population that is growing at a worrying rate and without strong action from Government the problems caused by this rapid rate of increase will continue," said Ms Joyce.