Achaidh Leithdeircc  Achaidh Leithdeircc (modern spelling "Achadh Leith-dheirg") is an ancient location in Ireland reputed to be the site of a historic battle, or series of battles, around the year 331AD, in which the forces of the Three Collas along with men of Connaught eventually conquered vast tracts of territory from the tribes of the Ulaid. The prehistoric royal site of Emhain Macha was burnt and plundered and ultimately this territory would re-emerge as part of the kingdom or federation of Airgíalla. The Battle of Achadh Leith-dheirg was fought in the territory of "Fearnmhagh", according to 19th-century Celtic scholar John O'Donovan. The place of the battle has been disputed between Fearn-mhagh, County Monaghan  and Aghaderg, near Loughbrickland, County Down. The 17th-century historian Geoffrey Keating in his "History of Ireland", in a stylized narrative of events, states: The Annals of the Four Masters, in the 19th-century translation by John O'Donovan (scholar), in a more perfunctory manner states: "The age of Christ 331. The battle of Achadh-leithdheirg, in Fearmhagh, [was fought] by the three Collas against the Ulstermen"... and in a note adds, "This place, situated in the territory of Fearnmagh, now the barony of Farney, in the county of Monaghan, has not been identified." A footnote in "Lebor na gCeart", published by the Celtic Society in 1847, contains a more expansive commentary regarding the location of Achadh Leith-dheirg: In summary, the predominant facts that emerge from any review of the battle, are that the Three Collas were victorious in the battle(s) of Achadh Leith-dheirg in "Fearnmagh" and that they drove the Ulaid beyond the Glen Righe. The Three Collas took from the Ulaid the land west of Glen Righe and Loch nEathach (Lough Neagh). At the Glen Righe, the Three Collas stopped and returned westwards to destroy Emhain Macha. The Three Collas did not invade or ever hold the land east/beyond the formidable boundary of the Glen Righe.