Donnchad Ua Cearbhall

Battle of Achadh Leith-dheirg
Battle site 331AD of the Three Collas & the Ulaid. Site of battle disputed between the three locations marked.
Date 331 Location Fearn-mhagh Result Three Collas victory
Belligerents
The Ulaid	The Three Collas
Commanders and leaders
King Fearghus Fogha	Colla Meann, Colla Uais, Colla Da Crioch
Battle site disputed between: Aghaderg parish - Barony of Dartree - Barony of Farney.

Achaidh Leithdeircc (modern spelling Achadh Leith-dheirg) is an ancient location in Ireland reputed to be the site of an historic battle, or series of battles, around the year 331AD, in which the forces of the Three Collas along with the Connaught men 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 Collas and the Connaughtmen... marched to the Carn of Achadh leth-derg, in Fernmagh. From that hill, they fought seven battles against the Ulstermen, that is, a battle on each day during a whole week. Six of these battles were fought by the Connaughtmen and the seventh by the Collas. In it Fergus Fogha, King of Emhain, was slain, and the Ulstermen were finally routed. They were then pursued with great slaughter, from the Carn of Eocaidh to Glen Righe. From that latter place, the Collas marched back upon Emhain, which they plundered, and then burned, so that Emhain has remained without a king to inhabit it.

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."

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Wikipedia