The Heritage Council has joined An Taisce in appealing to An Bord Pleanala against Offaly County Council's decision to approve a £130 million development at Durrow Abbey, near Tullamore.
In its appeal the council, an independent statutory agency, said the plans were premature because "the nature and extent of the archaeological and architectural significance" of the early monastic site remained unknown at the present time.
Describing Durrow Abbey as a site of major international significance, the council said the scale and character of the scheme put forward by Radleigh Developments Ltd would overwhelm such visible monuments as the abbey's 9th-century high cross.
Radleigh, run by a Roscrea-born developer, Mr John Maher, is seeking to build a 105-bedroom hotel, 475 houses or apartments, an 18-hole golf course and Champneys-style leisure centre on the 300-acre estate.
The director of the National Museum, Dr Pat Wallace, has strongly criticised the scheme and four appeals in the names of individual objectors have been lodged.
The council said Durrow was the only monastic site in Ireland identified definitively as having been founded by St Columcille. It had been an important Columban monastery, a place of burial of Irish kings and the site of an Augustinian abbey.
An inscription on the high cross records its erection, probably by Maelsechlainn. The site also contained the base of a second cross and a Norman defensive motte. The Heritage Council said Durrow had to be treated as "a complex of monuments of varying dates which are set in a relatively undisturbed landscape".
What lay underground, notably the line of the monastic enclosure, was equally important.
The council cited a policy document, Framework and Principles for the Protection of the Archaeological Heritage, issued last year by the Minister for Arts and Heritage, Ms de Valera, which laid down new ground rules for developments affecting archaeological sites.
"There should always be a presumption in favour of avoiding developmental impacts on the archaeological heritage", the document said, and `only the most compelling reasons could justify removing or moving ancient monuments.
Under the Durrow Abbey scheme none of the visible monuments would be directly affected. Indeed, the developer has agreed to hand over the core monastic site and an area around the nearby holy well to Duchas, the Heritage Service, and car-parking space.
But the Heritage Council said Radleigh had not put forward a design which would avoid impacting on the site, nor had the company considered relocating elements of its scheme or provided compelling reasons to justify the development.
