Planning permission to transform the Central Mental Hospital into an extensive housing development is to be sought within a year.
More than 1,200 private, social and affordable homes will be built on the walled site in Dundrum, Dublin, many through renovation of the existing Famine-era buildings.
A design team of Irish and international experts has been chosen to get the ball rolling on the project, which has been in the offing for a decade.
Their task is to come up with a scheme that will open up one of the city's least seen sites which, since 1850, has been surrounded by five-metre high security walls.
Inside, the original Victorian hospital building shares 28 acres of grounds and mature gardens with an array of more modern facilities.
Irish firm Reddy Architecture and Urbanism will work on the project with international design consultancy Tyréns and other Irish firms.
The Land Development Agency (LDA) announced their appointment while setting out an ambitious timetable for the €400m project.
It said construction would begin in 2022, presuming permission was granted. The project will go directly to An Bord Pleanála under the Strategic Housing Development provisions and tenders for building contractors will follow.
The Central Mental Hospital cares for mentally ill prisoners as well as people who have been charged with a serious criminal offence but are deemed too ill to plead and those tried for serious crimes but found not guilty by reason of insanity.
Patients and staff remain on site as they await transfer to a new facility on the site of the former St Ita's Psychiatric Hospital in Portrane, Co Dublin.
Work at Portrane is nearing completion and the move is expected to take place later this year. In the meantime, the LDA said it had access to the Dundrum site for surveys and evaluation work "with due regard for the sensitivity of the CMH's operations".
John Coleman, LDA chief executive, said the opportunity to work on the unique site was exciting.
"We plan to transform the Dundrum and Windy Arbour area for the better with a landmark sustainable and inclusive new neighbourhood of in excess of 1,200 new homes, incorporating significant period buildings and mature landscaped grounds," he said.
The LDA was set up to find state land for 150,000 homes over the next 20 years. It is currently active on nine sites with capacity for 4,000 homes.
Legislation has not yet been passed to put the agency on a permanent footing and there has been opposition to the 60pc weighting in favour of private housing, but it is expected it will be preserved in its current form under the next government.