The Collas first went to their kin in Connaught [King Muiredeach] and there gathered a great army for the invasion of Ulster. Fir Rois was located in south Airghialla. On the plain of Farney (Fernmag) in Monaghan they met the Ulstermen under their king, Fergus, and on seven successive days broke battle upon them, finally slaying Fergus and putting the Ultach (Ulstermen) to complete rout. Of the conquered portion of Ulster, from Louth in the south to Derry in the north, and from Loch Neagh to Loch Erne, the Collas made themselves the new kingdom of Oirgialla (Oriel). During a similar period the southern branches of the Airghialla came under the dominion of the southern Uí Néill kingdoms of Mide and Brega. During the thirteenth century the Anglo-Normans gradually reclaimed land from the Liffey so as to obtain a deeper berthage for merchant ships. In 1595, Ó Neill officially endorsed the inauguration of Brian Mac Hugh Og as 'MacMahon' and supported an invasion of Farney that restored Ever Mac Con Uladh to his ancestral home.
The Irish Annals record for the yeare of our Lord 331, "The battle of Achadh Leithdheirg, in Fearnmhagh (Farney), was fought by the three Collas against the Ulstermen, in which fell Fearghus Fogha, son of Fraechar Foirtriun, the last king of Ulster, who resided at Eamhain. They afterwards burned Eamhain [Macha], and the Ulstermen did not dwell therein since. They also took from the Ulstermen that part of the province extending from the Righe and Loch nEathach (Neagh) westwards. Colla Meann fell in this battle.
Colla da Crioch - The first king of Arghialla would be Colla-da-Chrioch (aka Colla Fochríth). From Colla da Chrich, were descended the MacMahons, princes of Monaghan, lords of Ferney, and barons of Dartree, at Conagh, where they had their chief seat. The MacMahons (MacMathghamhna) were sometimes styled princes of Orgiall. The MacMahons succeeded the above Chiefs, as Lords of Corca Baisgin. An interesting account of the MacMahons, of Monaghan, is given by Sir John Davis, who wrote in the reign of James the First. It may be observed that several of the MacMahons in former times changed the name to Mathews. The other chief clans of Monaghan were the MacKennas, chiefs of Truagh; the MacCabes; the MacNeneys, anglicised to Bird; the MacArdells; MacCassidys; O'Duffys, and O'Corrys; the O'Cosgras, MacCuskers or MacOscars, changed to Cosgraves, who possessed, according to O'Dugan, a territory called Fearra Rois, which comprised the district about Carrickmacross in Monaghan, with the parish of Clonkeen, adjoining, in the county of Louth; the Boylans of Dartree; the MacGil Michaels, changed to Mitchell; the MacDonnells; the O'Connellys, and others.
From Colla-da-Chrioch were also descended the MacGuires, lords of Fermanagh, and barons of Enniskillen; the O'Flanagans of Fermanagh; the O'Hanlons, chiefs of Hy-Meith-Tire, now the barony of Orior in Armagh, who held the office of hereditary regal standard-bearers of Ulster; the MacCathans or MacCanns of Clan Breasail, in Armagh; the O'Kellys, princes of Hy Maine, in the counties of Galway and Roscommon; and the O'Madagans or O'Maddens, chiefs of Siol Anmchadha or Silanchia, now the barony of Longford, in the county of Galway; The Cenél Cairpre (Cairbre) - northern Co. Sligo and northeast Co. Leitrim. The ancient kingdom of Meath was no inconsiderable principality, for it comprised the present counties of Meath and Westmeath, with parts of Dublin, Kildare, King's County, the greater part of Longford, and small portions of the ancient districts of Brefny and Orgiall on the borders of the present counties of Cavan and Louth. Uaithney was the name of the Eoghanacht cantred here, controlled by the O'Maoilriain (O'Mulryan or O'Ryan) and St. Malachy who were cited as chiefs of 'Owney Mulryan' which included both Owneybeg, and Owney in Co. Tipperary. O'Dinan (Dinnahane) is cited as chiefs of Uaithne here, as hereditary chiefs of the Eoghanacht Uaithne Agamar.
In 1140 Maelmhadhog O'Morgair, better known as St. Malachy, the great reforming bishop of Down and at one time Archbishop of Armagh was travelling to Rome. On the way to Rome, he visited Clairvaux, and was so touched by St. Bernard and the life at Clairvaux that he was converted to the monastic life. St Malachy founds Ireland's first Cistercian monastery at Mellifont where Donough O`Carroll, the King of Uriel, was the chief benefactor of the monastery. He was next induced by the papal legate Gilbert to accept the archbishopric of Armagh when there was a significant break in the line of hereditary coarbs (erenaghs) long dominant there. In 1111 he held a great synod at Fiadh-Mic-Aengus at which were present fifty bishops, 300 priests, and 3,000 other ecclesiastics, and also Murrough O'Brian, King of southern Ireland, and his nobles. During his incumbency the priory of Sts. Peter and Paul at Armagh was re-founded by Imar, the learned preceptor of St. Malachy. Two kings gave it their royal sanction: Felim, King of Munster, in 822, and the famous Brian Boru, in 1006.