Permission to continue work on the controversial site at Carrickmines has been given by the Minister for Environment, Heritage and Local Government, Mr Cullen, and Dún Laoghaire-Rathdown County Council.
Yesterday's announcement will clear the way for the continuation of the south-eastern motorway section of the M50.
Announcing his decision Mr Cullen said it was "satisfactorily demonstrated that a systematic approach" had been used by Dún Laoghaire-Rathdown County Council for "the archeological resolution of the Carrickmines area".
Over two years of archeological excavation has already taken place at the site. The Minister said work will now continue "to preserve the main archeological elements of the site either by record or in situ".
The Minister received 23 submissions relating to the case, 17 supporting early completion of the project and six objections. "The Minister has given careful and detailed consideration to this issue," a spokesman for the Department said last night.
"He even visited the site himself and would have studied and considered each of the 23 submissions and all material relating to the project," he said.
"This consent will strengthen up the archeological aspect of this site and future projects.
"The archeological aspect will be front of mind in dealing with these issues at all times," he added.
Joint consent was required under the National Monuments Act 1930 in order to interfere with the archeological site at Carrickmines Castle, originally demolished in 1641.
It must now go before both Houses of the Oireachtas for 21 sitting days to be debated from the start of the autumn sitting of the Dáil. The director of transportation at the county council, Mr Eamonn O'Hare, said the decision would be welcomed in the area by many.
"There's a significant groundswell of people in favour of this. The local community in Sandyford and business in the area are being seriously damaged by the lack of a motorway," he said.
"In Dundrum, Ballinteer, Sandyford there will be continued traffic chaos until the motorway is completed as planned," said Mr O'Hare.
Mr Vincent Salafia, the spokesperson for the Carrickmines Castle Preservation Group, said Mr Cullen had "signed the death warrant for the M50" yesterday.
"We have two different legal teams going to challenge his joint consent decision," said Mr Salafia.
"The consequences at European level are going to be very serious.
"The only reason they're listing is exigency. This decision goes against the very spirit of the Monuments Act," he said.
"There is a conflict of interest here, the Minister has in effect approved his own consent.
"He could have gone ahead with the motorway and bended the interchange but instead he bullied his way," Mr Salafia added.
Speaking on behalf of An Taisce, Mr Ruadhan MacEoin told The Irish Times that they felt there were "missed opportunities in relation to the Carrickmines Castle site".
"This decision is unfortunate given that there are issues raised by Carrickmines Castle and the location of the M50 junction under investigation" said Mr MacEoin.
"We fear that this decision will lead to further delays and further costs," he said.
Mr MacEoin said their compromise plan put forward was to let the motorway go ahead but with a redesigned junction.
"Why was this compromise not seriously assessed?" he asked.