The incident took place at Lisnabulrevey, near Fintona in Co Tyrone, when attempts were made to remove the rath, a circular fortified settlement dating from the Iron Age, the Irish Independent has reported.

In Irish folklore the monuments are sometimes called fairy forts because some believe they were occupied by fairies. There are more than 2,000 of them in Northern Ireland.

Police moved to halt work at the site last Saturday.

The PSNI said: “Police received a call from a concerned member of the community on September 2 stating that they believed the ring fort was being removed. Police attended and intervened and also liaised with the Department for Communities (DfC), who are investigating the matter.”

Statutory responsibility for heritage sites lies with DfC.

When asked who was responsible for causing what the department described as “considerable damage” to the fort, it said it would be inappropriate to comment.

While Lisnabulrevey Rath is not one of the 1,900 monuments protected under law, it is on the list of 16,000 historic sites contained on a DfC database, the Northern Ireland Sites and Monuments Record, meaning consideration is supposed to be given in planning policies when changes are proposed.

DfC added: “Officials have visited the site and advised that no further works take place at the rath.

“Officials from the Department of Agriculture, Environment and Rural Affairs are also aware of the incident and discussion is ongoing between DfC, DAERA and the local planning authority.”

Sorcha McAnespy, an independent member of Fermanagh & Omagh District Council who lives close to the site, said: “I am very disappointed to hear this.

“This is a very important area of heritage rooted in Celtic mythology. It’s known locally as the ‘fairy fort’.

“There are protection mechanisms at local council level but I suppose you cannot legislate for people moving in. Most examples of these forts are in the north of Ireland.”

Ms McAnespy said the damage demonstrated a lack of consideration for cross-border link-ups in the tourist industry.

She said: “There doesn’t seem to be enough emphasis placed on encouraging tourism based on this heritage.

“I have spoken on this extensively, in that there is no strategy that could link our local heritage to the Wild Atlantic Way or Ireland’s Ancient East, programmes that have already been established.”