Afterwards, he is not mentioned in any of the annals of Ireland until the year 1075 when he was defeated by the Kingdom of Airgíalla in battle near modern Ardee in County Louth, taking heavy losses.

This was probably Muirchertach's first major political appointment as it was in this year that he was first mentioned in the Annals of Tigernach, signifying the power and the importance of the position he held.

In 1088, Muirchertach led a number of naval campaigns against Ruadhrí na Saide Buide Ua Conchobhair, King of Connacht.

However, a new rival emerged in the form of Domnall Mac Lochlainn, King of the Northern Uí Néill, who later in 1088 invaded Connacht and successfully gained the submission of Ruadhrí na Saide.

He marched with his army back home to Munster on foot, but using Muirchertach's confiscated ships, the Meathmen and Connachtmen sailed southward on the Shannon and plundered the plains around Cashel.

Following this conference, Muirchertach went on campaign in Leinster again, but while he was away, Munster was attacked by Ua Maél Sechnaill who was joined by Ruadhrí na Saide.

Muirchertach took advantage of this situation by invading the province and gaining the kingship, in the process possibly taking in and grooming his nephew Tairrdelbach Ua Conchobhair for the day he would become king of Connacht.

He was involved in the Isle of Man as well, more or less controlling the island, possibly through one his sons, Tadc, brother of Muirchertach, marrying a woman from the Manx royalty.

With a fleet of around sixty ships and several thousand men, he re-established Norse power in the area, garrisoning the islands of Orkney and Man.

[5] The work was intended to romanticize the accounts from the annals, in a bid by Muirchertach to secure the O'Brien Dynasty's claim to the High Kingship, in a time when the throne was fiercely contested by the Northern Uí Néill in Ulster and later, the Ua Conchobhairs of Connacht.

The six-week long campaign began with his army marching north to the River Erne at Assaroe, then to the Inis Eoin Peninsula, burning Ardstraw and Fahan on the way.

The offensive culminated with the destruction of Grianan an Aileach, an important fort of the Northern Uí Néill, in revenge for Ua Lochlainn's earlier burning of towns in Munster.

With the Ulaid now under his control, however, he had another angle from which to attack the Uí Néill from in the Northeast, and easier access to the Hebrides and other areas of the Isles, and parts of Scotland.

Like many of the High Kings of Ireland from the South had found, including Muirchertach's great-grandfather Brian Boru, the last place to recognise their authority was Ulster, namely the Kingdoms of Ulaid and the Northern Uí Néill.

At the turn of the century, Muirchertach led annual campaigns against Domhnall Ua Lochlainn and the other northern kings, with mixed success.

Around the same time, Muirchertach was also under threat from Magnus Barefoot of Norway, who had returned with a larger force than his first Irish Sea campaign, possibly with the intention of invading Ireland.

Magnus had earlier raided Inis Cathaigh (Scattery Island) at the Shannon estuary in 1101, possibly testing the situation and defenses of Ireland.

[7] In the summer of 1103, Muirchertach launched an offensive against Domnall Ua Lochlainn, but was defeated on 5 August at the Battle of Mag Coba.

[8] It is also possible that powerful men in Norway wanted Magnus removed from the throne, and bribed a contingent of his forces to desert back to their ships during the battle.

He resolved to kill Arnulf himself as a reward for his alliance, but the latter ... fled to his own people and lived for twenty years afterwards with no fixed abode.

"[13] According to Orderic Vitalis, Muirchertach only allowed the Normans to stay in case he needed their support to fight Magnus if he were to violate their treaty and attack the Irish.

However, after the Norwegian King's death at the hands of the Ulaid, Muirchertach turned against the English rebels and forced them out of Ireland, possibly in an attempt to make amends with Henry.

Indeed, Henry had imposed sanctions on Irish trade for their assistance in helping the rebels, but Muirchertach was able to negotiate the lifting of the embargo (possibly by handing Arnulf over to the English), while also again demonstrating his skill in diplomacy.

The marriage between Muirchertach's daughter and Arnulf went ahead regardless, and later, the High King wrote to the English bishop Anselm of Canterbury, thanking him for intervening with Henry on behalf of his son-in-law.

Either way, it is evident Muirchertach's influence reached as far as Scotland, that Edgar viewed the High King as either a valuable ally or a serious threat, and that peace with him was the best option.

[3] By 1100, Muirchertach controlled almost all of the island, apart from the north-western territory of the Northern Uí Néill who, under the leadership of King Domnall Ua Lochlainn continued to remain defiant.

In the period of 1101–1112, he, like his father Toirdelbach Ua Briain, presided over the Synod of Cashel, and was able to influence reforms in the Church, notably relating to marriage.

However, conflict with Ua Lochlainn flared again in 1112 when Domnall defied Muirchertach by marching south to Dublin, where he burned Fingal and carried away many captives and herds of cattle.

Muirchertach apparently went into retirement having lost his High Kingship, and died at Lismore, modern County Waterford, a year later in 1119.

Altogether, Muirchertach Ó Briain lifted his head above the domestic power struggle and sought to involve Ireland in the international politics of Europe, so that some sixty years later, these activities were still well remembered, and are reflected in the vitae of St. Flannan of Killaloe.