It is generally held that the Déisi Muman pushed north-west starting from the 5th to the early 8th century, taking the area from the Uí Fiachrach Aidhne of the Connachta.

In a bid to secure the High Kingship of Ireland for the clan, Muirchertach encouraged ecclesiastical reform in 1111 with the creation of territorial dioceses over the entire island.

They had support for their bid from several foreign connections including the Norwegian king Magnus Bareleg and the Anglo-Norman baron Arnulf de Montgomery, who were both united to the clan through marriage in 1102.

[6] MacLochlainn's plans to restore the High Kingship to the north was thwarted by his ally Tairrdelbach Ua Conchobair of Connacht who formed an alliance with the O'Brien's.

Domhnall was a man of realpolitik; his main concern was upholding his position in Thomond and was not against collaborating with Strongbow and others against rival Gaelic kingdoms such as Ossory, Desmond and Connacht.

[7] Domhnall even acknowledged Henry II as Lord of Ireland at Cashel in 1171, but a mere two years later when Plantagenet tried to grant Thomond to Philip de Braose this situation was overturned.

[citation needed] After the death of Domhnall Mór a period of destructive feuding among his offspring caused a great territorial decline in Thomond.

[citation needed] The Normans' advance continued during the reign of Conchobhar na Suidaine Ó Briain, as Henry III "granted" lands to Robert de Muscregos and John Fitzgeoffrey.

[7] The following year, the Gaelic chiefs from all over Ireland had resolved to form an anti-Norman pact and met at Cael Uisce, near Lough Erne to discuss terms.

Tadhg was proud and stubborn, declaring his father should be High King; the Dál gCais thus did not take part in the Battle of Druim Dearg, which the Gaels lost, damaging the reputation of Thomond as a consequence.

[citation needed] Brian Ruadh Ó Briain kept the pressure up by burning Clare Castle and forcing the de Muscregos' to flee to Wales, but he had troubles from his own countrymen.

Seeing an opportunity, Edward II offered Thomond to Thomas de Clare if he could take advantage of the Ó Briain feud.

Mathghamhain Maonmhaighe Ó Briain came to power before the death of his uncle Diarmuid and he was named as such because he spent time as a foster child in Máenmaige.

This had the added benefit of winning back Limerick for Thomond and Sioda Cam Mac Con Mara was placed in the city as a warden in 1369.

Thomond was now in such a position that Conchobhar mac Mathghamhna Ó Briain's twenty six-year reign was marked as a time of peace and plenty.

During this time, Mathghamhain Dall was deposed by his brother Toirdelbhach with the familiar military assistance of the de Burghs (whom he had formed a marriage alliance with).

Mac Fhirbhisigh hints that the men of Leinster planned to raise Tadhg to the High Kingship of Ireland before his death and claims he was the greatest Ó Briain since Brian Bóruma himself.

[citation needed] Thomond was wealthy in the 15th century; Domhnall Mac Gormáin (died 1484) was described as the richest man in Ireland in terms of live stock.

Toirdelbhach's life came to an end trying to defend the Ó Cearbhaill of Éile from the Earl of Ormond at Camus, near Cashel; he died "by the shot of a ball.

In 1534, a rumour had spread that his father, the Lord Deputy of Ireland, had been executed in England on the orders of king Henry VIII and that the same fate was planned for him and his uncles.

In hot pursuit, at the head of an army, was Lord Leonard Grey, who destroyed the Killaloe Bridge, which had the result of isolating Thomond from the rest of Ireland and also attacked the Dál gCais east of the River Shannon.

[citation needed] Although the Silken Thomas issue was resolved by late 1535, Thomond had marked itself out by providing refuge to enemies of the Crown of England in Ireland.

The English forces had in turn enlisted the services of Conchobhar's own son Donnchadh Ó Briain who had cemented an alliance with the Butler family by marrying the daughter of the Earl of Ormond.

The Parliament of the Kingdom of Ireland was called to Limerick in 1542 by Lord Deputy Anthony St. Leger regarding the terms of submission of Murchadh Carrach Ó Briain and Sioda Mac Con Mara.

Tied into English political rivalries, Conchobhar had the support of the Earl of Sussex but was not able to decisively defeat his uncle, indeed Domhnall scored a victory at the Battle of Spancel Hill in 1559.