The Irish Penal Reform Trust has claimed that plans for two "super" prisons were the equivalent of the Bertie Bowl for the Minister for Justice, Mr McDowell.
Mr Rick Lines, executive director of the trust, condemned plans for the new facilities to replace Mountjoy and Cork prisons, and claimed they were being used to mask the fact that the Department of Justice planned to increase prison spaces by 1,000 without any justification.
Responding to the criticisms, Mr McDowell said he was surprised that the trust, which had called for the replacement of Mountjoy and Cork Prisons, was objecting to his plans.
Mr Lines claimed, however, that the Minister's proposals for more prison spaces would lead to Ireland having "the fourth largest per capita prison population in western Europe".
He accused the Department of Justice of "confusing two issues": the need to replace outdated prisons with the question of prison numbers.
Conditions at Mountjoy and Cork had been condemned by various inspection reports, he said. The Government had obligations under international law to provide adequate prisoner accommodation, said Mr Lines.
"This is a very separate issue and is very separate and distinct from the incarceration rate... The Department of Justice is trying to disguise and deflect the debate on the incarceration rate by wrapping it up in a humanitarian approach," he added. "It's being used to deflect a broader discussion as to whether it makes sense to spend money on building more prison spaces."
Mr Lines questioned the need to increase prison spaces, citing figures that two-thirds of prison committals were for non violent offences, often for possession of drugs or failure to pay fines.
Mr Lines was commenting following a report in yesterday's Irish Times which revealed that the Irish Prison Service has shortlisted six 100-acre sites on the fringes of Dublin for a replacement prison complex for Mountjoy. The site eventually selected is also expected to accommodate the Central Mental Hospital.
Yesterday, the Prison Officers' Association criticised Government plans for replacing Mountjoy as unnecessary. It claimed that the older parts of the prison, some of which are 150 years old, should instead be completely renovated. A spokesman for the association said that more than 70 million had been spent in recent years on new facilities in the prison, including a new women's prison, which would now be demolished under the current proposals.