According to both oral and written traditions, at the beginning of the 4th century A.D., three brothers, known as the Three Collas, rebelled and killed their uncle, the high king of Ireland, Fíacha Sroiptine. This singular barbarous act started a chain of fateful events that would have a devastating and far reaching impact on the northern half of the island. After his uncle’s death, his nephew, eldest brother Colla Uais, took Fíacha’s kingship and ruled for three years (323-326 AD). He also laid claim to the overall kingship, and ruled as Ard ri (high-king) of Ireland for four years. At the end of that time the murdered king’s son, Muiredach Tirech banished the Three Collas, and exiled them and three hundred of their followers to Alba (Scotland).

The old annals of Tigernach tell us that the three brothers returned to Ireland, and traveled to Connaught where many of their relatives still lived. They approached the local king Muiredeach and with his help gathered a large army with the sole intention of raiding the province of Ulster. In 331 AD on the open plain of Farney, in what is modern day County Monaghan, a final, bloody battle was fought. The battle lasted for seven days and resulted in the death of Fergus Foga, the king of Ulster, and the routing of his armies. The royal seat at Eamhain Mhacha (Navan Fort,) the ancient capital of the province of Ulster, was burned down and destroyed. The victors then claimed the area as their own and re-named it Oirghíalla (Oriel.)  

The name Oirghíalla means "the hostage givers" and the new kingdom was comprised of nine minor-kingdoms, each named after their ruling families. During its heyday,  Oirghíalla ruled the modern dioceses of Armagh and Clogher, and controlled parts of counties Armagh, Monaghan, Louth, Fermanagh, Tyrone and Derry. Its main seats of power were the towns of Armagh and Clogher. Oirghíalla stretched from Louth in the south to Derry in the north, and from Loch Neagh to Loch Erne. The new kingdom would last for almost a thousand years and go on to play a vital part in the history of the province of Ulster.

The New Kingdoms.

1 The Uí Tuirtri, meaning "descendants of Tort" were based east of the Sperrin Mountains in eastern County Derry and Tyrone.

2 The Uí Maic Cairthinn, meaning "descendants of Cairthend" settled south of Lough Foyle in north-western County Derry.

3 The Uí Fiachrach Arda Sratha, meaning "descendants of Fiachrach of Ard Straw" were based at Ardstraw in modern-day County Tyrone.

4 The Uí Cremthainn, based in what is now parts of modern-day County Fermanagh, Monaghan, and Tyrone.

5 The Uí Méith, based in modern-day County Monaghan.

6 The Airthir, meaning "Priestly” were based around the city of Armagh, and held control of the offices of the church in Armagh, which had pre-eminence in Ireland.

7 The Mugdorna, based originally in County Monaghan, re-located to southern County Down, and named the area after themselves. Their name lives on as "Mourne", the present-day name for the area and the Mourne Mountains.

8 The Fir Chraíbe, also known as the Fir na Chraíbe, meaning "men of the branch" were located west of the River Bann in north-eastern County Derry.

9 The Fir Lí, also known as the Fir Lee, meaning "people of Lí" were located west of the River Bann in mid-eastern County Derry.

Another dynasty, the Uí Moccu Úais were composed of members of three tribes namely, the Uí Tuirtri, the Uí Maic Cairthinn and the Uí Fiachrach Arda Sratha, and were collectively known as the Uí Moccu Úais, eventually moved their bases to Counties Meath and Westmeath and near to the old kingdom of Brega.

The territory of the Oirghíalla from the 6th century onwards was gradually eroded by their neighbors, the northern Uí Néill, as well as the southern Uí Néill to their south. From 735 AD they were fully controlled by the Cenél nEógain, and by 827 AD had become their minions. The kingdom of Oirghíalla was at its zenith in the 12th century, under king Donnchad Ua Cerbaill. The later smaller kingdom of Airgíalla survived in Monaghan and was re-named Oriel after the Norman Invasion of Ireland. The kingdom of Oirghíalla came to a final end in 1585 when Rossa Boy MacMahon agreed to surrender and regrant his territories to the English Crown in Ireland, with his territory becoming County Monaghan in the Tudor Kingdom of Ireland.

Rossa Boy had ascended to the Oirghíalla kingship in 1579 and immediately found himself in an unfavorable position, wedged between an ever-expanding Tudor kingdom and Tír Eoghain under the control of the O'Neills. At first, he made overtures which suggested an alliance with Tír Eoghain, when he married the daughter of Hugh O'Neill, Earl of Tyrone. However, hoping to be left alone to run affairs locally, and after pledging allegiance to monarch Elizabeth I, MacMahon met with John Perrot, then Lord Deputy of Ireland, who according to some, was a natural son of the Tudor monarch Henry VIII, and agreed to join their Kingdom of Ireland. Oirghíalla, now known as Monaghan, was divided into five baronies under native Gaelic chiefs, mostly from the MacMahon clans.

This was not the end of the matter however because, fearful of the English moving in closer to his own lands, Hugh O'Neill turned to Brian Mac Hugh Og MacMahon of Dartree and married off another of his daughters to him. At the time, Brian Mac Hugh Og was the Tanist (leader) of his people according to the native Brehon laws and O'Neill was hoping to bring the Oirghíalla clans back to his side on the death of Rossa Boy through this agreement. For his part, Rossa Boy was trying to establish a pro-English succession through his brother Hugh Roe MacMahon. When the new Lord Deputy, William FitzWilliam began to pressure the acceptance of an English High Sheriff of Monaghan, O'Neill used his influence to exact opposition to it from clansmen in Monaghan, Leitrim, Fermanagh and Donegal. This prompted a military force led by Henry Bagenal, to be sent into the county in early 1589 to install the new sheriff and by the summer of that year, Rossa Boy McMahon was dead. In less than two decades all traces of the power of the Oirghíalla and the O’Neills would be consigned to the dusty pages of history.

An excerpt from "Out of the Ice: Ireland Then and Now."

By Author / Poet John A. Brennan.

Available at: https://www.amazon.com/author/johnabrennan

Below is a list of some of the sources I accessed:

The Book of Invasions

The Annals of the Four Masters

Keating’s History of Ireland

The Book of Leinster

The Annals of Ireland

Annals of Ulster

Annals of Boyle

Annals of Connacht

Annals of Inisfallen

Annals of Tigernach