Justice Minister Dermot Ahern today opened a new wing at Dublin's Wheatfield Prison catering for up to 200 prisoners.
The new block includes 176 cells and will bring the capacity of the jail to more than 600 inmates.
Mr Ahern said the expansion is the first stage in rolling out extra space in prisons across the state.
"The provision of these facilities represents just one aspect of the major prisons capital programme funded by the Government," Mr Ahern said.
"This new block in Wheatfield is part of an investment programme in additional prison spaces being rolled out by the Irish Prison Service."
Mr Ahern was forced to defend his handling of overcrowding in the prison service last week after a riot by inmates at Mountjoy in the capital.
The minister rejected claims conditions were to blame after 74 prisoners barricaded themselves into an exercise yard and armed themselves with makeshift weapons for two and a half hours on Thursday night.
But the Irish Penal Reform Trust (IPRT) claimed conditions at Mountjoy were hindering the work of officers.
The Irish Prison Service said the country's prison population reached more than 4,400 last week.
Mr Ahern said the new three-storey facility at Wheatfield, in Clondalkin, includes six wheelchair accessible cells on the ground floor, a segregation unit for prisoners requiring more specialised management, a new medical centre to cater for the entire prison and areas for recreation and office accommodation.
The minister also said a new accommodation block would be opened in the Portlaoise/Midlands prisons complex, with an extra 300 spaces, by 2012.
He said work would begin on converting an administrative building on the Dochas site, the state's women's prison, into a new accommodation block which will provide 70 spaces by the end of this year.
Tenders will also be issued for a further 150 spaces in Castlerea Prison and Shelton Abbey.
He reaffirmed the Government's commitment to developing the new super-prison at Thornton Hall.
"Work is already under way on the construction of the dedicated access route and the Irish Prison Service hope to award the contract for the installation of the off-site services to serve the prison development in November," Mr Ahern said.