Anti-roads protesters attempting to preserve the ruins of Carrickmines Castle have been granted a judicial review by the High Court.
Work to remove some of the remains of the medieval castle in south Dublin resumed earlier this month.
The group's grounds for a renewed judicial review were:
It argues that the Minister for the Environment had no right to permit the destruction of the remains for two reasons.
The first was that he was both guardian of the remains and proponent of the proposed motorway.
The second was that the enlarged motorway roundabout approved by the Minister is some 50 per cent larger than that approved by An Bord Pleanála and so would require a new planning application.
It argues that if the Minister did have the right to permit the works, it was only having determined that the ruins represent a threat to health and safety, which the group says is not the case.
The third ground for a renewed judicial review is that the complaint process to the EU is not complete and the work may be destroying evidence required for this case.
Archaeologists who resumed work on the castle's defensive fosse returned to continue numbering and moving stones from the walled fosse.
Mr Michael Egan, corporate affairs spokesman for the National Roads Authority, said that without a new court challenge, the road and its Carrickmines interchange would be open by September 2005.
The authority had previously considered opening the motorway and not the Carrickmines section, effectively leaving a gap in the M50 C-ring around Dublin.
It had also considered in recent weeks the possibility of isolating the main section of the motorway, and awaiting court decisions on the question of the interchange.
The review is to take place in January.