Preliminary excavations are beginning today ahead of the construction of the M3 motorway through the historically sensitive Tara-Skryne valley in Co Meath.
Minister for the Environment Dick Roche sanctioned 38 archaeological digs in the valley last month despite widespread opposition at home and abroad.
An Bord Pleanála approved the route in 2003.
Mr Roche refused last month to comment on a timeframe for the construction of the road, saying the excavations required would be "onerous", costing at least €30 million to €40 million.
It is expected the excavations could last about two years, with construction taking at least another three.
Mr Roche acknowledged opposition to his decision from National Museum director Dr Pat Wallace, from whom he had received a statutory report.
Dr Wallace wrote a detailed letter to Mr Roche as part of the Minister's consultations. This advised the Minister to issue a preservation order for the whole area around Tara.
Mr Roche has said his issuing of any orders would have been an interference with the already-concluded planning process.
The Save Tara and Skyrne valley campaign expressed misgivings today. "We are not pleased at all that excavations have started today because this signals the start of the Government trying to push the M3 through this important historic area," Muireann Ni Bhrolchain, spokeswoman for the campaign, told ireland.com.
She was also concerned that the company in charge of the excavations did not fully understand the importance of the site.
"They say they will get the job done in six to 12 months. It’s not realistic to be able to properly do this kind of work in such a short timescale.
She described the project as "rescue archaeology", where the aim is to remove as much as possible from the site in as short a period of time as possible. "This is not the in-depth research that the site calls for."
Among the conditions imposed are that all archaeological works should be done by professional archaeologists in advance of road construction; all work must be inspected by State archaeologists with regular progress reports; artifacts uncovered be sent to the National Museum; and additional time and/or money must be made available if required for excavations.
In a separate move, environmental activist Vincent Salafia is to take a High Court case in a bid to block the planned route of the M3.
Mr Salafia is meeting his legal advisers over the coming days to discuss the case, which will be centred on environmental and heritage legislation.
It is expected that the legal team will seek leave from the High Court this week to take judicial review proceedings against Mr Roche’s decision to allow for archaeological excavations to begin.