The Sunday Tribune can exclusively reveal that the Department of Justice has purchased an additional 6.7 acres at 150,000 per acre for the development. The land purchase adds to the spiralling cost of the Thornton Hall prison project just weeks after this newspaper reported that the state had spent another 1.3m on a small strip of land to provide a second access route to the site. The Thornton Hall site was purchased for 30m, which represents 200,000 an acre, two years ago.

A spokesperson for the Department of Justice explained: "The intended purpose of the 6.7 acres is for a number of criminal justice facilities including the new Garda Forensic Science Laboratory, the Garda Technical Bureau and a Garda station and possibly some other secure storage facilities. The final details have yet to be worked out and we are at a very early stage in the process."

Meanwhile, representatives of the Irish Prison Service have met with members of the residents' group in recent weeks as they attempt to allay residents' fears following years of acrimony and protests.

The Service also plans to issue a newsletter updating locals on the super-prison's progress.

The statutory public consultation procedure relating to the super prison development, as set out in the 2007 Prisons Act, is due to commence within weeks.

Teresa McDonnell, spokeswoman for the Rolestown St Margaret's Action Group said: "We are surprised to hear that the Department of Justice has bought more land at Thornton Hall as they already have over 150 acres for the prison.

"They said three years ago that 50 of the original 150 acres was not needed and would be there for further development.

"The purchase of these extra lands just opens the floodgates and we do not know what to expect in the future.

"We have never been concerned about a prison being built in our area. Our main concern has been the plans for a major development in what is a rural agricultural area."