Nineteen archaeological sites, including a Neolithic settlement and an early medieval cemetery, have been found along the route of the next and final stage of the upgrading to motorway standard of the Dublin to Newry road.
Construction of the 14-km high-quality dual carriageway route is under way. It is due to open in 2007 to the 25,000 motorists who travel the route each day.
The archaeological testing was carried out by Archaeological Development Services Ltd (ADS). Final excavations are almost completed.
An early medieval farmstead or enclosure was located at Faughart Lower on a small natural rise in the landscape which, in addition to views over Dundalk bay, gave it what ADS described as "a commanding position over an ancient routeway.
"Initial results suggest it is an early medieval (AD 400- 1169) enclosure that had been expanded several times before being finally used as a cemetery for the wider local community."
A souterrain or underground passageway measuring 40 metres was also found. It appears to have been deliberately de-capped in ancient times and the passageway filled in. The site is also home to a large number of early Christian graves, about 700 of which have been identified.
There was a Neolithic settlement in the townland of Plaster consisting of two rectangular structures, probably houses, each measuring about 9m (29½ft) by 6m (19½ft).
At Aghnaskeagh there is a cairn, recorded in the Louth Archaeological Survey as an "unclassified megalithic tomb", which lies partly in the road-take.
It dates from between circa 4000 to 2500 BC, and while about 40 per cent of the site lies outside the road-take and will be preserved in situ, the remaining 60 per cent which is affected by the new road is under excavation.
The road, which will cross the Border, will cost just under €122 million and will take traffic from the end of the Dundalk Western bypass, which opened last month, from the Ballymascanlon roundabout on the northern fringes of Dundalk to Cloghue roundabout on the southern approach to Newry, Co Down.
The discovery of the sites has not affected construction and councillors in Louth have been told the project remains on target. Although the link road is not being constructed as a motorway, it is expected to have a speed limit of 120km/h.