A pair of adjoining forts near Templemore, Co Tipperary, the probable site of an important 12th century church synod, are believed to have been destroyed.
A local archaeologist, Mr Donal O'Regan, said a raised square fort and "classic" ring fort seemed to have been levelled. He said he was concerned at the rate of destruction of ring forts in the area. Under the 1994 National Monuments Act, such actions are illegal.
"This is the traditional site of the Synod of Rathbreasil, which was held in AD 1111," he said. "It was the most important synod of the Irish church in the 12th century, the one which divided up the dioceses as we have them today. The balance of probability is that it was held at this particular site."
Locals say that until recently the outlines of buildings could be seen on the site, plus a number of graves. The owner of the land could not be contacted last night.
The synod, called by the King of Munster, Muirchertach Ua Briain, a great grandson of Brian Boru, was presided over by Bishop Gilbert of Limerick.
Mr O'Regan said: "There is a fairly high destruction rate of ring forts in this particular area. Since 1960, we have lost about 12 ring forts. These sites are now meant to be protected under national legislation and they are being bulldozed."
Nationally, this issue was highlighted in 1999 when a report for the Heritage Council found a third of the State's archaeological monuments had been destroyed since 1840.
Prof Barry Raftery, of the department of archaeology in UCD, said the scale of destruction was horrific and prosecutions were rare. "It is a finite resource," he said. Although the motivation was different, Prof Raftery said the principle was the same as the Taliban's destruction of Buddhist monuments in Afghanistan. "It is a part of our cultural heritage which cannot be replaced." Prof George Eogan, also of UCD, said farming practices were responsible for the destruction in many cases.
A pilot survey of south Meath, published in Riocht na Midhe this year, showed that a third of monuments were destroyed over a 25-year period.
"I am amazed it has not been rectified. The powers that be have fairly good legislation to back them up. The problem may be that there is not enough personnel," said Prof Eogan.
A spokesman for Duchas, the Heritage Service, said a complaint had been received at the chief archaeologist's office and would be investigated. "It is our policy to seek a prosecution where a monument has been destroyed knowingly," he said.