It seems to have been Claire o' Kelly (Illustrated Guide to Newgrange, 1978) who was the first to suggest that the linear feature to the east side of Newgrange, was a cursus. The monument (though I don't know if that's really the right word!) is more than 100 yards long and consists of a "pair of parallel banks with internal ditches about 20 metres apart, orientated north-south (thus sharing an alignment with the passage of the large tomb), ending in a U-shaped southern terminus."
Reference:
Newman, Conor, 'Notes on four Cursus-like Monuments in County Meath,' in Barclay & Harding's Pathways and Ceremonies, Oxbow 1999.