The book, Conall:Rinn-Iru
mentions a great earthworks - the Black Pig's Dyke.
Long before the Romans thought of building several walls to keep out the Picts and around the same time that the Great Wall of China was built, the ancient people of Ulster, the Ulaid, built the Black Pig’s Dyke or Gleann na Muice Duibhe, which is literally the “glen of the black pig”.
The dyke itself is a series of discontinuous defensive earthworks built along the southern boundary of the province of Ulster. Remnants of the banks and ditches stretch through South County Down, County Armagh, County Monaghan, County Cavan, County Leitrim and South County Donegal.
Excavations reveal that the original construction was of a substantial timber palisade with external ditch. Behind the palisade was a double bank with intervening ditch. The timber structure was radiocarbon dated to 390-370 BC, so the whole of Black Pig’s Dyke may date to that period.
However, the tales from old Gaelic folklore that the dyke was the result of a large boar who tore up the Irish country side with its huge tusks or that it was made by a huge worm wriggling across the countryside, are less factual, but perhaps much more interesting.