Carvings have been discovered on one of the stones of Grange Stone Circle at Lough Gur in Co Limerick.
Archaeological photographer Ken Williams made the discovery when he was checking the stones for carvings as part of his current research.
He has developed methods of lighting stones to photograph them to maximum effect and he has found a number of new carvings over the years in this way.
The new carvings have concentric circles and arcs placed on the back and sides of the fourth stone to the north of the entrance passage, referred to as Stone 9 in the published site plans.
The Lough Gur development was founded in 1969 and officially inaugurated in 1993 to promote, preserve and protect the history, archaeology, folklore and environment of the surrounding area.
Dr Elizabeth Shee Twohig, who has published extensively on megalithic and rock art, said the carvings are quite like those at passage tombs in the north and east of the country, such as Knowth and Newgrange.
She said there is only a single carved stone of this kind in Munster or Connacht.
"It is possible that the stone is contemporary with the banked enclosure henge at c.3000 BC and was incorporated into the circle built inside the enclosure at a slightly later date," she said.
Minister of State for Heritage and Electoral Reform Malcolm Noonan welcomed the discovery at Grange Stone Circle.
"This is a site that has both captivated and intrigued locals and visitors for many years.
"It shows yet again the capacity our national monuments have to surprise and engage us," he said.
Local TD and Minister of State at the Department of Public Expenditure and Reform Patrick O'Donovan thanked Ken Williams for notifying the Office of Public Works and the Department of Housing, Local Government and Heritage as he said: "This ensures that the discovery can be appropriately recorded."
He also encourage members of the public to contact the OPW and the Department immediately if they find anything unusual, as he said "everyone can play their part to protect our past".
Mr Williams plans to publish the photographs of the carvings in an academic paper, which will describe the art and discuss comparable examples elsewhere.