A large underground passageway was destroyed shortly after its discovery on lands where Louth County Council is to grant planning permission for 363 houses.
Standing orders were suspended at a meeting of the county council at the request of Cllr Donal Lynch who said the souterrain was "something of national and historical importance and has been demolished".
It was found in a field between Haggardstown and Blackrock. Mr Lynch added: "This is a shame, disgraceful and has to be put in the public arena." He asked where the debris has been taken and what action will be taken against those responsible.
The souterrain was found by local man Mr Pat Rafferty while out walking his dog and a small notice stating it had been discovered was published in two local newspapers last Wednesday. The next morning it had been destroyed.
"I saw a small hole in the ground and went down into it. It was very well built with flags of stones and very well constructed," he said. A sketch was made of the souterrain and it was found to be around 52 feet long. The floor was about 4.5 feet below ground and shards of pottery and flint were found which will be given to the National Museum.
The survey and sketch were done on September 10th and the site was destroyed the next morning.
Souterrains are underground enclosures and their use remains a mystery, although they are believed to have been either refuges in times of attack or for storing food.
Around a mile away a number of souterrains were found two years ago by archaeologists working on an extension to the Dundalk Institute of Technology and they are a common feature in Louth.
The souterrain was on land where planning permission had been granted for 363 houses. A council spokesperson said machinery on the site was doing exploratory work and the council had not been aware of the souterrain.