Senior archaeologists from its National Monuments section are liaising with gardaí in Co Cork as part of the probe.
It was launched following works on farmland in the village of Kilmurry near Macroom, Co Cork, on which the two recorded monuments were located.
Ring-forts are oval or circular fortified settlements or farmsteads that were built mostly during the Early Christian and Iron Age periods.
They date from about 600-BC to about 1,000-AD and some were still inhabited up until the 1700s.
They were owned by wealthy individuals who built houses and kept cattle inside the earthen ditches.
The wealthier the individual, the more ditches were built around the outside.
There are about 100,000 such structures recorded across Ireland and the two at the centre of this investigation were considered among the region’s finer examples.
One was oval and measured almost 60m in an east-west direction, 48m in a north-south direction, and was enclosed by a two-metre high earthen bank.
Archaeologists had found the remains of cultivation ridges crossing its interior.
The other ring-fort was circular and slightly smaller, measuring just more than 33 metres, and was surrounded by a two-metre high earthen ditch. It featured numerous cattle gaps across its bank.
However, both structures have been completely levelled. No above-ground trace remains. All their earthen banks have been removed and filled in.
Under the terms of National Monuments Legislation, landowners are required to give at least four week’s notice to the Department of the Environment about their intention to carry out works near recorded monuments. This did not happen in this case.
The Friends of the Irish Environment group has now written to Environment Minister John Gormley calling for the full weight of the law to be brought to bear in this case.
“While the vast majority of farmers and land owners have the greatest respect for our archaeological heritage, often at their own expense, there remain elements in the farming community who believe that they can destroy these sites at will because of the wide-spread historic lack of enforcement,” spokesman Tony Lowes said.
“The full weight of the law must be brought to bear in this case. The message must go out across Ireland that however few these individuals are, they will not be tolerated and the national heritage will be protected.”