The Irish Penal Reform Trust (IPRT) has "grave concern" as Irish prisons reach capacity and inmates are forced to sleep on mattresses on floors.
The total number of people in prison in Ireland now exceeds the number of beds available.
The official figures, published on the Irish Prison Service website, showed the combined capacity across prisons in the Republic of Ireland has now hit 100%.
One prison - Limerick (female) - is now at 164% of its capacity.
In total, 4,416 people were in prison across the country at the beginning of the week, with only 4,411 beds available.
The IPRT is calling on the Minister for Justice to make "immediate efforts" to 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.
Acting Executive Director Molly Joyce said: “The Government has indicated that they are committed to exploring the policy of ‘prison as a last resort’ through providing alternatives to prison such as community-based sanctions where appropriate. The policy position is there but we can see by the growth in the number of people in prison that the policy is far from reality on the ground.
"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.”
IPRT is further concerned that the continued delay in publishing the Prison Chaplain Annual Reports for 2021 misses an opportunity to shed light on what is happening on the ground in Irish prisons.
Previously published chaplain reports have highlighted various issues within the prison system, including the impact of overcrowding
Ms Joyce added: "The number of people in prison can be reduced now by careful and structured use of temporary and early release measures. In the longer term, measures to reduce Ireland’s prison population need to be adequately resourced and implemented.
"There continues to be an over-reliance on imprisonment for people convicted of less serious offences, despite its damaging social and economic impact on individuals, families, and communities.”