Dúchas, the Heritage Service, has confirmed that a 3,000 year-old cooking pit within Dún Aengus on the Aran island of Inis Mór was damaged last year. It occurred during last year's foot-and-mouth disease restrictions when the site was officially closed.
A large stone which formed part of the cooking pit was removed at a time when entrance to the fort or cashel was banned. Not even Dúchas staff were permitted to enter during the nine-week period when the foot-and-mouth disease precautions applied, and it was some time before the missing stone was noted.
Described as one of the most important prehistoric monuments in Europe, Dún Aengus dates from about the first century BC. The cooking pit was discovered during digs there between 1991 and 1995, led by Ms Claire Cotter and a team of archaeologists.
The excavations unearthed proof of human habitation inside the fort, which comprises a semi-circular structure on the edge of a 270-foot-high cliff overlooking the Atlantic.
The missing stone was the last remaining of four stones which would have lined the pit.
A spokesman for Dúchas told The Irish Times that it is believed that it may have been dumped into the sea and is not retrievable. The Heritage Service is now looking at obtaining a suitable replacement, he said. The restrictions which banned Dúchas staff from entering the fort - where access is now through a Dúchas heritage centre - were a matter for the Department of Agriculture, he said.
During those restrictions, tourist operators on the island were known to have taken visitors to another access point - by-passing the visitors' centre, which also charges a small fee.
Dúchas staff are constantly present within the fort to ensure that small stones from the walls are not removed and to guard against graffiti - which has despoiled part of the structure in recent years.
Last year, a prominent archaeologist in the west, Mr Michael Gibbons, criticised Dúchas for lax "heritage practice" in relation to protection of the fort. He said that part of the fort was damaged during construction of an emergency metal fence as part of the foot-and-mouth restrictions.Dúchas denied the claim.
A temporary fence to keep animals away from adjoining land proved to be ineffective, and a fixed fence was installed in the bedrock. Mr Gibbons said stones were taken from the fort's chevaux-de-frise to build a link wall with the metal fence.
The distinctive chevaux-de-frise comprises splinters of basalt, which are set at an angle to bar access. It forms an integral part of the Dún Aengus structure, and suggests that the fort was a military monument. Not only was it disturbed, but the bedrock was also damaged by the metal fence, Mr Gibbons said.