April 23rd is the 1,000th anniversary of the death of Brian Boru, the most famous and, arguably, the greatest Irishman before the modern era. He died in his hour of victory at the Battle of Clontarf, after a lifetime of remarkable achievement.

The man we call Brian Boru – Brian of Béal Bóraimhe, near Killaloe in Co Clare – was from a Munster family formerly of no great distinction. He was born into an Ireland that was hidebound by tradition, where political power was dominated by a single great dynasty, the Uí Néill.

These descendants of the eponymous and perhaps mythical Niall of the Nine Hostages occupied a vast swathe of Ireland, their southern branch inhabiting the midlands from the Shannon to the Irish Sea, the northern Uí Néill ruling from modern Donegal to the River Bann.

For half a millennium, until Brian came along, the Uí Néill had held exclusive rights to the almost mystical “kingship of Tara”. Over the course of time, and certainly during the ninth and 10th centuries, an equation came to be made between possession of this arcane trophy and exclusive entitlement to the kingship of all Ireland.

This meant that, although at any one time upwards of half a dozen provincial rulers were powerful enough to bid for national supremacy, the force of tradition and Uí Néill propaganda combined to deny them the honour.

This is why Brian Boru was a hugely important figure, even before his triumph at Clontarf. He flouted this convention. For 20 years this upstart led a political and military struggle to undermine the ruling dynasty. And by the year 1002 Brian had succeeded in forcing the Uí Néill high king, Máel Sechnaill mac Domnaill, to recognise him as his superior.

It was a landmark in Irish history. Brian had demonstrated that one did not have to be a descendant of the eponymous Niall to put forward a claim to the high kingship.

So he fundamentally altered the rules of Irish politics at the start of the 11th century – undermining the Uí Néill monopoly so that the high kingship of Ireland was now up for grabs – and this remained the case for a century and a half, until the high kingship went into abeyance soon after the Anglo-Norman invasion.

The death of Brian Boru, the new high king, in triumph at Clontarf in 1014 ended what the great Irish medievalist Edmund Curtis called "the Norse tyranny". This perception of the battle as a watershed in Irish history is of long standing. One of the most valuable compilations of medieval Irish annals is manuscript 1293 at Trinity College Dublin, otherwise known as the Annals of Loch Cé . The literati behind it had access to early-medieval material, but the manuscript begins only with the year 1014, meaning that even in the Middle Ages it ranked as a key milestone.

The manuscript’s account of the Battle of Clontarf, more than 1,200 words long, is more the stuff of romance than history, but evidently the compiler viewed Brian Boru’s victory as possibly the single most glorious achievement by any of his nation.

That attraction remains. Brian has been, for generations of Irish people, the hero who led his people to victory over their would-be Viking conquerors and secured their freedom from foreign oppression, so much so that of the 50 or so high kings who reigned from the dawn of Irish history until the institution was smothered, in the aftermath of the English invasion, Brian is (a little depressingly) the only one who commands general recognition today.

Martyr's death?
Brian was also a Christian king portrayed (however inaccurately) as facing heathen opponents. And because the Battle of Clontarf happened to take place on Good Friday there is more than a suggestion that he died a martyr's death, becoming the saviour of his people just as Christ saved humankind by His Easter sacrifice.

It is sometimes assumed that this interpretation is a modern nationalist construction. But it has been there from the start. The Irish monk Marianus Scottus, who wrote a chronicle in Germany in the decades before his death, in 1082, says that Brian, the king of Ireland, was killed on April 23rd, 1014, while preparing for Easter, having both “hands and mind focused on God”; he therefore died at prayer, not at war.

This is replicated in independent Icelandic sources, which have it that Brian didn’t actually take part in the battle because he refused to fight on Good Friday.

This idea of a Christ-like king laying down his life for his people had become fixed by the 13th century, when the poem beginning “Aonar dhuit a Bhriain Bhanba” (“To you alone, O Brian of Ireland”) was written. Attributed to Muiredach Albanach Ó Dálaigh, it has the quatrain:

On Good Friday Brian was killed
Defending the hostaged Irish . . .
Just as Christ without sin was killed
Defending the children of Adam.

And the climax of the poem has a message for a later generation of Irish people:

When will there come the like of Brian
South or north, east or west,
Who will protect the Irish against evil
As he alone protected?

Even in the Middle Ages Brian was depicted as saviour of the Irish, his death a noble sacrifice. But a recent process of questioning has aimed to expose the “myth” of Brian’s expulsion of the Vikings.

The first trenchant assault came from a University College Dublin professor of medieval Irish history, Fr John Ryan. As far back as 1938 he published an essay in which he claimed that Clontarf had nothing to do with Brian losing his life defending Ireland from a Scandinavian assault. He argued instead that the battle was little more than the culmination of a rebellion against Brian, the king of Munster, by the mutinous king of Leinster and his Dublin sidekicks.

This view has gained wide currency among historians. Few now, other than purveyors of popular fiction, portray Brian as eliminator of the Viking scourge.

It is correct that the Vikings, who began to attack Ireland in the closing years of the eighth century, and who had constructed some stable settlements here, including Dublin, by the middle of the ninth, were not “banished” forever from Ireland when defeated in the Battle of Clontarf.

It is also the case that the descendants of the Scandinavians, who founded Dublin and the other Hiberno-Norse towns, continued to live in and govern them long after 1014.

And the battle indeed featured an alliance of Leinstermen and Viking forces, pitted against Brian’s Munster army.

But that doesn’t mean it was a mere local skirmish. Something about the events of Good Friday 1014 cannot be explained away as the resolution of a quarrel between the king of Munster and insubordinate Leinster and Dublin rebels. Such an interpretation offers no basis for viewing Clontarf as a victory, and few doubt Clontarf was a victory for Brian, even though he forfeited his life in it.

There is a danger that we have talked down the significance of Clontarf too much.The truth is that the battle was an international contest. Neighbouring England was conquered twice in the 11th century, by the Normans in 1066 and by the Danes just months before the Battle of Clontarf. It is no coincidence.

Their leader was Sweyn Forkbeard, who forced England’s Anglo-Saxon king Ethelred the Unready into exile. (Although Sveinn died suddenly in February 1014, King Ethelred being temporarily restored, this only momentarily halted the succession of Sweyn’s son Canute.)

So the Scandinavian fleet that descended on Dublin Bay in late April 1014 did so in tumultuous circumstances. It had seen that prosperous England could be conquered in a matter of weeks – and, just as quickly, temporarily lost – and no doubt had high hopes of similar conquests in Ireland.

Instead it came up against the army of Brian Boru, and there followed what the Irish annals call the “slaughter of the Foreigners of the Western World” at Clontarf. That slaughter is what earned Brian his lasting reputation and why, despite his death, his side considered Clontarf a wondrous victory.

It is not the case that the Battle of Clontarf was primarily the climax of Leinster’s rebellion again Brian Boru. Brian marched on Dublin in the autumn of 1013 and laid the city under a siege; he marched on it again in the spring of 1014 because Scandinavian Dublin was the focus of resistance to him.

Yes, the Leinstermen threw in their lot with the Dubliners, but there is no evidence that the Leinstermen dominated Dublin at this juncture. Instead our sources emphasise that King Sitric Silkenbeard of Dublin was Brian's chief adversary, having joined forces with other ambitious Scandinavian figures, such as Earl Sigurd of the Orkneys.

There is no telling, at this remove, what precisely King Sitric and his allies hoped to achieve, but, if England could be won and lost, Ireland surely seemed a sitting duck.

And the question that was asked of Brian on that Good Friday, 1,000 years ago this month, was whether he would crumple as Ethelred the Unready had done in the face of Sweyn Forkbeard the previous summer. That he did not was his claim to fame. That he gave his life to the task was his remarkable achievement.

Seán Duffy is a fellow of Trinity College Dublin and author of Brian Boru and the Battle of Clontarf (Gill & Macmillan)

Clontarf 1014-2014: A National Conference to Mark the Millennium of the Battle of Clontarf is at TCD tomorrow and Saturday. Admission is free