From Irish Roots, 1996, #3, p 22, col 3:
The O'Brien name will be forever linked with the town of Killaloe because it was there that Brian Boru had his palace of Kincora. The O'Briens were of the clan of Dal gCais . . . Originally to be a Dalcassian meant that you came from the area around the border of Clare and Tipperary but nowadays it is used to cover all of County Clare.

He was slain in the battle of Clontarf.

From A Concise History of Ireland by Maire and Conor Cruise O'Brien, p 34:
". . . [it was] about 859 that an acknowledged effective High King emerges, in the person of Maelsechnaill, a prince of the Ui Neill. Irish kingship, however, being not hereditary but elective, the paramountcy of the High King was strictly personal and had to be reasserted in arms by each new claimant, so that the long dynastic struggles were to continue as much a factor in the anarchy of the times as any Viking presence. . . . The Ui Neill retained their pre-emption of the High Kingship for a relatively short time; the essentially disparate character of the power-structure militated against it. Their rise had created a vacuum in Munster and a hitherto obscure people, the Dal gCais, made themselves masters of the province. One of their princes was Brian Boroimhe (Boru), and by 1002 he was undisputed High King of Ireland. His reign is unique not only because of his vertiginous rise to power, but because of its extraordinary effectiveness. He saw himself as an Irish Charlemagne and thought in terms of institutions for the whole island. He confirmed Armagh in the ecclesiastical supremacy of the country, north and south. He restored communications and protected learning. The eleventh century was one of recovery and reform and the credit for their initiation is traditionally his. The final phase of the Viking wars may be taken as ending with his defeat of their forces at the Battle of Clontarf in 1014, where, at an advanced age, he met his death. Significant is the fact that the Norse in this battle were the allies of the perennially rebellious Kingdom of Leinster, who alone among the Irish local sovereignties could claim they had resisted Brian's overlordship."

Book "A New History of Ireland" vol IX, Moody, Martin & Byrne, eds. ISBN 0-19-821745-5, p 137: King of Munster 978-1014, High King of all Ireland 1002-1014

From Irish Roots, 1994 #1, page 27, 'Surnames of County Galway':
Maolruanna O'Heyne, a king of the south Galway region, was slain in 1014. [At Clontarf?] His daughter, Mor, was the first wife of High King Brian Boru.

From the book "The Oxford History of Ireland" edited by R. F. Foster, 1992, ISBN 0-19-285271-X, page 37:
On his death notice in 951 Cennétig, king of Dál Cais, is called 'king of North Munster'; his son Brian Bóruma, or Brian Boru (ancestor of the O'Briens), who succeeded in 976, was to make himself king of Ireland and rival of the Uí Néill. The rise to power of Brian Boru was swift. He succeeded his assassinated brother and rounded on his killers, among them the Ostmen of Limerick. Their king Imar and his sons, all Christians, took refuge in the monastery of Scattery Island. Brian desecrated the church and killed them in sanctuary. IN 3 years, Limerick and Munster were in his hands, and he came into conflict with the Uí Néill when he tried to conquer Ossory in 980. Allied with the Ostmen of Waterford (he needed their fleet), and holding the petty kingdoms and the monastic towns of Munster in a tight grip, he pushed his armies into Connacht and Leinster. Mael Sechnaill's attempts to contain him did not
page 38:
succeed, and in 997 Brian and Mael Sechnaill met at Clonfert and divided Ireland between them. This made Brian master of Dublin and Leinster. Late in 999 the Leinstermen and the Dubliners revolted against Brian; but he marched north and inflicted a crushing defeat on them at Glenn Máma, and took the king of Leinster prisoner. He followed this up with a winter war against Dublin; he plundered the city, burned the fortress, and eventually forced Sitric Silkenbeard, king of Dublin, into submission. Now Brian set out, with caution and determination, to dominate the whole country-and in this attempt he was consistently supported by his new subjects, the Ostmen of Dublin. This was done by 1011. And he sought the blessing of the church: in 1005 he spent a week in Armagh, formally recognized it as the apostolic city of Ireland, placed 20 ounces of gold as his offering on the high altar, and had himself described in the Book of Armagh as 'Emperor of the Irish.' His success changed the rules of the game entirely. He had shattered the Uí Néill monopoly and had made the kingship of Ireland a prize to be fought for by the greatest of the provincial kings. Brian Boru was killed at the battle of Clontarf on Good Friday 1014. About 1012, relations between Brian and Leinster became strained, and there was a general reaction in Ireland to his rule. Late in 1013, Brian's son was sent to attack Leinster, which he spoiled as far as Glendalough, and he camped at Kilmainham within view of Dublin. He was joined by Brian and together they blockaded the city from Sep to Christmas, but without success. The Leinstermen and the Ostmen knew they would be back and attempted to put together a large force of Vikings from the Western Isles and Man to help them. The battle which followed was unusually bloody: Brian won, but he himself was killed, as were many of the leaders on both sides. Clontarf became a great battle of saga and literature-for the Ostmen as well as the Irish. It was not a struggle between the Irish and the Vikings for the sovereignty of Ireland as people have often thought. Nor was it a great national victory which broke the power of the Vikings forever-that did not need to be done. In fact, Clontarf was an important event in the struggle for dominance between the great provincial rulers: Brian was stopped short just when he seemed about to make the kingship of Ireland a reality and the possession of his dynasty.

Brian mac Cenneidigh, born in the province of Munster around 941 AD, was the youngest of 12 brothers, all but two of whom would be killed in battle. Members of the Dal Caissan clan, the brothers fought continuous wars against the Danes and Irish rivals from Leinster. When his brother, Mahon, became King of Munster and eventually undertook a treaty with the Vikings of Ivar, who was the Norse king of Limerick, Brian Boru then waged guerrilla war against the Vikings from his base in the Thomond mountains. When the Vikings broke their treaty, Brian lead an army that defeated the Vikings at the Battle of Sulchoid in 968. (He then followed the Vikings back to Limerick. Where he was so angry to find large numbers of enslaved Irish children that he executed three thousand Vikings in revenge.)

Two remarkable kings lived in Ireland; Malachy, King of Meath and Ard Righ of Ireland; and Brian Boru, King of Munster. Brian was, at this time, coming into prominence because of his ability and his successes against the Danes. In the election of the Ard Righ, or High King of Ireland, the most powerful Provincial king was sometimes chosen. A jealousy existed between Malachy and Brian, who afterwards succeeded in dethroning Malachy and becoming Ard Righ. Brian first became King of Thomond, North Munster; he next became Provincial king of all Munster; then he claimed sovereignty over the southern half of Ireland, Leath Mogha. This claim of Brian excited the jealousy of Malachy; if Brian ruled Leinster and Munster he might become too strong for Malachy. Therefore it was a struggle between the rest of Ireland under Malachy, against Munster, which led to the Battle of Nenagh. In that age it was the custom to inaugurate the kings of Munster under a great oak tree at a place near Tulla in the County Clare, called Magh Adhair, now Moyre. This tree Malachy maliciously cut down, and had it taken up by the roots, using the timber of the oak to roof his palace. Brian determined to avenge the insult. He led an army in boats up the Shannon, entered Meath, and burned the royal rath of Dun Sciath. The timber of Brian's oak was in the roof of this rath and Brian had the satisfaction of burning the timber which Malachy had cut down. This act of Brian was the immediate cause of the Battle of Nenagh. Malachy was absent in Connacht at the time; he re-crossed the Shannon, marched south, and burned Aenach Thete. After this victory Malachy attacked the Danes near Dublin, and carried away the collar of gold of Tomar, a Scandinavian chief. Three years after the Battle of Nenagh, Brian and Malachy were united, to the great joy of the Irish. Their combined forces attacked the Danes of Dublin in the year 998, and plundered a great part of their wealth, previous to the Battle of Clontarf. Brian was acknowledged by Malachy as King of Leath Mogha, Southern Ireland, and Suzerain of Leinster.8

Brian Boru was born around 940, the youngest of two sons of Cennedig, head of Dal Cais, one of the royal free tribes of Munster. Brian grew up during the worst days of tyranny when the Dalcassians had been driven in to the present county of Clare. Brian’s brother, Mahon, being the eldest, succeeded Cennedig as chief of the Dalcassians. Being hemmed into Clare by the Norse Leader, Ivar of Limerick, Mahon was willing to accept terms but Brian, seeing almost all of the Dal Cais tribe including his mother brutally murdered by a Norse raid when he was only a child, refused to be any part of such a truce. He deserted Mahon with a group of soldiers. They lived in the hills of Munster attacking Norse settlements and disappearing in to the hills. His fame spread throughout the province and infuriated Ivar. Although having only a handful of men, Brian’s skill as a tactician led him to defeat vastly superior numerical forces and led to rumors of a mighty Dalcassian army.
After a number of petty battles, Brian had trained an excellent Dalcassian army to face the Norsemen. The stories of his triumphs had led to vast numbers of young men volunteering to join his side. The feud between himself and Mahon ended. Mahon renounced his truce with the Norsemen and the two brothers rejoined forces. The two men triumphed so far that Mahon took the throne of Cashel in 963 and in 968 at Sulchoid in Tipperary, the two brothers completely overtook Ivar’s forces and marched on Limerick while Ivar fled back to the Norse lands. The Norse tyranny in Munster thus collapsed and Mahon ruled peacefully for eight years. However, Ivar returned to Ireland and plotted the murder of Mahon. After Mahon’s death, Brian not wanting a bloodbath between his forces and Ivar’s, honourably challenged Ivar to open combat, which he won killing Ivar. Brian succeeded his brother as head of the Dal Cais and immediately took the field against his brothers enemies. In 978, he defeated the King of Cashel in battle. Step by step he established himself in the Kingship of Munster and fortified the province. In 983 and 988, his fleets ravaged Connaught and plundered Meath.
Meanwhile, another great leader had arisen in the North, Malachy the second, the Ui Neill King of Tara. Malachy was born in 948, became King of Meath and in 980, High King. This he achieved at the battle of Tara in 980 where he overthrew a Norse Army and took Dublin. A clash between the two men was inevitable. At last, in 998, they met and divided Ireland between the two of them, Brian becoming the King of the South and Malachy of the North.
By 1002, the joint sway of Malachy and Brian could not last. Malachy, being unable to gather enough support to take on the mighty forces of Brian, allowed Brian peacefully to take over his lands. This was the greatest moment in the history of native Ireland. Brian, by his title, “Ard Ri”, was claiming the monarchy of the whole Gaelic race. Before Brian, and Malachy, Ireland was divided in to a number of petty kingdoms, sometimes at peace, sometimes at war with one another. The Vikings themselves joined in the struggles between the Irish kingdoms and also fought bitterly among themselves. There was no one king up to this who was responsible for the defense of Ireland against the Vikings and had control over the entire island.
Brian had much to do as High King to lift Ireland out of the ruins of the Norse Age. He rebuilt ruined churches, built others, he sent overseas to replace lost books and artifacts and all that he possibly could to heal the wounds of the past two centuries of Norse pillage.
In 1013, the Leinstermen and the Dublin Vikings revolted against Brian. Mael Morda, King of Leinster, allied himself with the Dublin Vikings and went to war with Brian. The Dublin Vikings sought allies overseas. The great Sigurd, Earl of Orkney, came with a large contingent. While other Viking contingents came from as far afield as Iceland and Normandy. Brian gave them Battle at Clontarf on Good Friday, 1014 and defeated them. However, as the Vikings were retreating, one of their leaders, Bothair, murdered Brian.
After this, Malachy resumed his position as High King and the Dal Cais strength remained only in Munster. The Viking presence in Ireland continued after Brian’s death but their military power was crushed. They remained in the country as traders and intermarried amongst the native Irish. Ireland was never again to have a King to control the entire of the island and the cost to Ireland and to Brian of crushing the Viking power in this country was a great one, for Ireland was never again to have a true “ARD RI”.

The battle of Clontarf Irish army, commanded overall by Boru, was composed of the division of Munster, the division of Meath, the division of South Connacht, commanded by Maolrunaidh O Heyne, the Scottish division comprising ten Mormaers or Earls of Scotland, including Donald the Earl of Mar, who was killed that day, and one Viking division from Northumbria. Against this army was Mael Morda, King of Leinster, who was in revolt against Boru, the army of the Danes under Sitric Silkbeard, King of Dublin, and the Vikings of Orkney, Man and the Hebrides under the command of Earl Sigurd the fat, earl of Orkney.9