The heritage watchdog body’s Mark Clinton claimed the site at Kinnagorva, near Clonea Strand close to Dungarvan, was of major heritage and cultural importance and was “totally unsuitable” for commercial development.
He is the chairman of An Taisce’s Monument and Antiquities Commission.
He urged the minister to act promptly over the application for the proposed development of the Co Waterford site.
A company, Lar Whelan Trading, acquired planning permission in 2008 for a mixed use development of the site, including 98 houses with 35 apartments, a hotel, modified village centre of four buildings, shop, crèche and a medical centre.
Permission subsequently granted by Waterford City and County Council for the replacement of 64 holiday homes with a 100-bay caravan park, has been appealed to Bord Pleanála.
A decision is expected in late September.
Dr Clinton’s letter said the exact status of the site had yet to be determined but enough evidence had already been unearthed to merit its conservation.
The letter refers to an early church having stood on the site and the discovery of five Ogham stones there in 1857, followed by two more in the 1960s, which are stored in Mount Melleray Abbey and in the National Museum of Ireland.
He said the five original Oghams may have originated in an ecclesiastical souterrain, as was indicated in papers in 1891 and possibly representing the church or monastic huts.
Dr Clinton has proposed a full excavation of the site.