An Taisce has written to the Minister for Transport, Mr Brennan, asking him to suspend further construction work on the Dublin South Eastern motorway until the archaeological heritage of Carrickmines Castle is protected.
The heritage group has also written to Minister for the Environment, Mr Cullen, asking him to place a temporary preservation order on the medieval site.
The final leg of the M50 motorway passes through the Carrickmines site in south Dublin. Work on the road has been continuing around the site but is scheduled to start there once an archaeological dig is wound up next month.
The significance of the castle site was not realised until the dig began almost two years ago. Archaeologists have found a well-preserved castle wall enclosing 1.5 acres of buildings, workshops, houses, kilns, wells and numerous ditches. Up to 20,000 pieces of medieval pottery have been recovered, as well as coins, cannon balls, weapons, skeletons and textiles.
The dig was expected to last six months, but now has an archaeological team of more than 100.
An Taisce's chairman, Mr Michael Smith, said the archaeological heritage of the site had not been adequately appraised before the road construction started. Therefore it did not comply with the EU Environmental Impact Assessment Directive, he said.
Earlier this year the director of the dig, Dr Mark Clinton, told The Irish Times that if the remains of Carrickmines Castle lay on a greenfield site unaffected by motorway plans, they would be preserved as a national monument.
The National Roads Authority said in June that the finds would not affect the routeing of the road. A Dúchas spokesman also said the road development complied with all the necessary requirements.
When An Taisce called for other options to be considered, an NRA spokesman said this was not feasible. An Taisce has asked for a feasibility study on a revised road and intersection alignment which would preserve the monument.
A spokeswoman for Mr Brennan said he had asked the NRA and Dún Laoghaire-Rathdown Council for a report and he would then be in a position to respond to An Taisce. A spokesman for the Minister said he wanted to see what emerged from Mr Brennan's investigations before commenting.
