The 150-acre site in north Co Dublin selected for a new prison complex to replace Mountjoy could cost millions more to excavate than the €30 million paid for it, according to leading archaeological experts.
Ten days ago the High Court granted leave to objectors to commission their own geo-physical survey of the site. If this confirms what is already known from aerial photography, an archaeological excavation "would cost an absolute fortune", one expert said.
Dr Mark Clinton, who headed a similar excavation of the Carrickmines Castle site in south Co Dublin, said the Thornton Hall site was part of a known archaeological landscape extending back to prehistoric times and "can't be isolated from it like an asteroid floating in outer space".
He said the archaeological excavation at Carrickmines involved a core area of three to four acres. The cost came to €6.5 million by the time it came to an end in 2003.
John Maas, an expert in analysing aerial photographs, said the much more extensive Thornton Hall site contained "layer upon layer of archaeology from continuous habitation, probably dating back to around 7,000 BC" and the State had "a duty under law to excavate it".
If the geo-physical survey sanctioned by the High Court confirmed his view that there was "an awful lot of archaeology there", Mr Maas said the cost of acquiring the site would "pale into insignificance" compared to the "many millions" required for a full archaeological dig.
Local residents opposing the proposed prison have already had a geo-physical survey carried out on adjacent lands in Kilsallaghan, and this showed it was a zone of rich potential. They also commissioned a report from Dr Clinton on the area's history and archaeology.
Apart from identifying 33 sites of archaeological interest, the report highlighted the importance of the Battle of Kilsallaghan in March 1642, when insurgents were defeated by royalist forces led by James Butler, then Earl of Ormonde, after a fierce struggle involving 5,800 men.
Two weeks ago, members of Fingal County Council unanimously adopted a Green Party motion designating the Thornton Hall site as an Architectural Conservation Area and initiating the process of listing the mid-19th century house on the council's Record of Protected Structures.
Minister for Justice Michael McDowell is on record as saying that he would not be put off building a prison by any "guff about fairy forts or architecture".
However, the Government's decision to develop the site is currently the subject of judicial review proceedings and also an official complaint to the European Commission.
