According to Whitaker, this county was inhabited in the time of Ptolemy by the Scoti, who then possessed all the inland parts of Ireland; it afterwards formed part of the district of Uriel, Oriel, or Orgial, which also comprehended Louth and part of Armagh; but it was more generally known by the name of Mac Mahon's country, from the powerful sept of the same name. Its present name is derived from its chief town, MONAGHAN or Miunechan, "the Town of the Monks," although no trace of an ecclesiastical establishment can now be discovered there. Immediately after the English invasion, when De Courcy entered Ulster, he was joined by a chieftain named Mac Mahon, who ingratiated himself so much with him that he was entrusted with the command of two forts, which, on the first change of fortune, Mac Mahon utterly destroyed; and when questioned on his breach of faith, answered, "that he had not engaged to keep stone walls; and that he scorned to confine himself within such cold and dreary enclosures, while his native woods were open for his reception and security."

Co. Monaghan. The Mugdorna territory stretched from Monaghan, where it is preserved in the name Cremourne (Crích Mugdorna "the territory of the Mughdorn"), south to as far as the river Boyne at Navan. As noted in entries in the Annals below, the territory of Mugdorna in the 9th and early 10th centuries was divided into Mughdorn Maighean and Mughdhorn Breagh, indicating a division of north and south, with Breagh referring to an area in or near eastern co. Meath. The early chiefs of Mugdorna are stated to be descendants of one of the Three Collas, i.e. Colla Mend.

In its history the area originally referred to as Mughdorna was eventually sub-divided into the smaller kingdoms of Uí Meith, Dartraige (now Dartry), Fir Fearnmhaigh (now Farney), Conailli (now part of co. Louth) , Fir Ros (now the area about Carrickmacross), and Mugdorna [now Cremorne]. The Ua Machainen sept are noted in the annals as lords of Mughdhorn in the 11th and early 12th centuries, probabaly at a time when the region of Mughdhorna was more limited in size, possibly centering on the barony of Cremorne in co. Monaghan. The Ua Machainen's (O'Mochoidhein) are also mentioned in O'Dugans famous topographical poem.

 

ARMAGH (Ard Mhacha) is supposed to have been part of that name by Ptolemy as the territories of the Vinderii and Voluntii: it afterwards formed part of the district called Orgial, which also comprised the counties of Louth and Monaghan. The Celtic Queen gave her name to the City Ard Macha, meaning ’The Height of Macha’, reflecting the fact that the city developed on the hill overlooking Navan Fort. Famed for being the ecclesiastical capital of Ireland, this claim dates back to AD 445 when St Patrick established his principal Church in Ireland. The formation of this part of Ireland into separate dominion is said to have taken place so early as the yeare 332, after the battle of Achaighleth-derg, in Fermoy, in which, as recorded by Tigernach, abbot of Clonmacnois, who died in 1068, Fergus Feagha, son of Froechair the Brave, the last of the Ultonian kings who resided in Emania, was killed by the three Collas, who then expelled the Ultonians from that part of the province to the south of Lough Neagh, and formed it into an independent state, to which they gave the name Orgial, afterwards corrupted into Oriel or Uriel, names by which it was distinguished to the beginning of the 17th century. The chief part of the county prior to the arrival of the English had centered in the families of the O'Nials, the Mac Cahans, and the O'Hanlons.

(Clan Cernaich) - The baronies of Oneilland East and West in modern county Armagh seems to have have derived their name from the ancient territory of Uí Nialláin. From an early date the Airthir kings of the Uí Nialláin sept ruled from Loch Cal (Loughgall) just north of Armagh. The Ua hAnluain (O'Hanlon) sept, among others, were noted as chiefs of Uí Niallain as early as the 10th century. They continued as chiefs in the area of the Oriors for the next few centuries.

Síl Daim Argait - The early genealogy is cited in Laud 610 as Daim Argait (Corpri) son of Echach son of Cremthaind Leith son of Feicc son of Dega Duirn son of Rochatha son of Colla Fochríth. Among the prominent groups included in Síl Daim Argait included Síl nDaimine and Clann Lugainn of the modern county Fermanagh area. Throughout the 11th and 12th centuries the Kings of Fermanagh - O'hEignigh, O'Maolruanaidh and O'Dubhdara - were drawn from the Airghialla, its Clann Lugainn branch, which is stated in the early genealogies to go back to one of the three Collas, i.e. Colla Fochríth. O'Dugan (Poems) mentions Uí hEignigh and Muintir Maoilruana as "two kings over the good slopes of Monach". He also cites a sept of OhEignigh over Clann-Cearnaigh, and apparently in connection with Uí Lorcain of eastern Uí -Breasail. O'Heany or Hegney (Ó hEignigh) and Mulrooney (Maolruanaidh) were noted as kings of Fermanagh (Fer Manach, or Fir Managh) until becoming tributary to the Maguires (Meicc h-Uidir of clann Lugain) by the 13th century.

The Cenél nAlbanaich, a Laigin tribe were a branch of the Oirghialla that settled in the northwest Highlands and Islands in very early times. Their chief clans descend from Godfraidh Mac Ferghusa (i.e., "Fergus"), a prince of the Oirghialla in North Ireland who came to Scotland, or Albany, in the ninth century as an ally of Kenneth MacAlpin, first king of the united kingdom of Picts and Scots. The Cenél nAlbanaich settled north of Argyle in the Hebrides, in the area of Skye, where they acquired Pictish and later Worse connections. The chief clans which branched from the Cenél nAlbanaich are the Clann Dhomhnuill and the Clann Dubhghaill. The Clann Dhomhnuill or MacDonalds (Mac Dhomhnuill) descend from Dhomnuill, or Donald, son of Reginald (or Ranald) mac Somerled, King of the Isles and Lord of Argyle and Kintyre (1164—1207). Ranald’s mother was the daughter of Olav, Norse King of Man and the Isles. It was from her that he derived his titles in the Isles, his paternal grandfather Somerled being already Lord of Argyle (the Lordship of the Isles was under the control of the King of Norway until 1266).

Northern Armagh. The O'Garvey (O Gairbhith) sept, from the same stock as O'Hanlon, held sway in Armagh on the southern shore of Lough Neagh (Oneilland East) before being displaced by the MacCann's, lords of Clanbrassil. The Annals for 1155 cite Amhlaibh Mac Canna as 'pillar of chivalry and vigour of Cenél Eoghain." Other noted chiefs included O'Keelaghan (O Ceileachain) of Uí Breasil Airthir. O'Dugan (Poems) cites the sept of O'Gairbhith fierce chiefs of Uí -Breasial of Macha, as well as the sept of Mag Duilechain over Clann Breasial. O'Dugan goes on to note the sept of Uí Lorcain over the high eastern Uí -Breasail, and also the septs of the O'Longains, O'Duibheamhnas, and O'Conchobhairs all of the western Uí -Breasail.

The area around Lough Ooney, aka Loch Uaithne near Smithborough in the barony of Dartry, co. Monaghan, was apparently referred to at an early date as Fernmag or Fer Fernmaighe (or part of). Later migration to southeast county Monaghan apparently brought the territorial name with it (Farney). An early genealogy of Fernmaigi goes back to one of the three Collas, i.e. Colla Fochríth (aka Colla Da Crich).

Coba vs. Arghialla: Were there two adjacent Uí Echach territories with two separate genealogies? The Uí Ethach Coba (Ui Ethach Uladh in the baronies of Iveagh, in co. Down) were a powerful group (of Dál n-Araidhe descent) in adjoining county Down, however the Book of Fenagh and Leabhar na gCeart make note of an Uí Echach (baronies of Iveagh, co. Down) as a sub-territory of Airghialla. In Place-Names of North Ireland, for County Down, it cites The Uí Echach were also known as the Uí Echach Coba to distinguish them from similarly named groups, to the east in the Ards peninsula (Ui Eachach Arda), and to the west in Airgialla.

The diocese of Dromore reflects the earlier boundary between Uí Echach Coba and Airgialla, in that it includes the Armagh parishes of Seagoe and Shankill and follows the river Bann all the way to Lough Neagh. Its founder is believed to be Colman, located midway between Downpatrick and Armagh. St. MacErc, a brother of Colman's teacher, Mochay, had charge of the church in Donaghmore in the fifth century and was a consequence of the reordering of the Irish Church by Rome in the twelfth century. The area that formed the small diocese coincided with the medieval baronies or territories of Upper and Lower Iveagh, what we might term today south and west Co Down. It also included a portion of Ó Neill land, that part of Co Armagh which lies east of the river Bann, and a tiny piece of south-west Co Antrim. A Cistercian monastery was established at Newry, dating from 1144.

The north-western boundary of the diocese follows the River Bann from Knock Bridge (on the modern boundary with Co. Armagh) all the way to Lough Neagh, and includes the district of Clanbrasil, now the barony of Oneilland East. Clanbrasil was traditionally part of Uí Echach but was annexed to the new County of Armagh in 1605.