Sigurðr was the Viking earl of Orkney, whose control extended to other parts of northern Britain as well. He was a ruler of considerable power in the region.
He fought against Brian Boru's army at the Battle of Clontarf alongside the forces of Dublin and Leinster and was killed during the battle.
A good deal of what we know about Sigurðr comes from Norse sagas written long after he died. Unfortunately, many of these later sources can be of dubious reliability unless they are approached carefully.
One of these sources is Njál's Saga, which describes how Sigurðr became involved in the events at Clontarf:
Then King Sigtrygg stirred in his business with Earl Sigurd, and bade him go to the war with him against King Brian. The earl was long steadfast, but the end of it was that he let the king have his way, but said he must have his mother's hand for his help, and be king in Ireland, if they slew Brian. But all his men besought Earl Sigurd not to go into the war, but it was all no good."
Sigurðr in Clontarf
According to Njál's Saga, Sitric of Dublin recruited Sigurðr, as well as various other overseas Viking leaders, for the Battle of Clontarf, by promising him great power in Ireland if they were to defeat Brian Boru. In a colourful albeit partisan account, appearing as it does in the Cogadh Gáedhel re Gallaibh compiled by Brian Boru's twelfth-century descendants, Sigurðr is described appearing for the Battle of Clontarf with an army of:
...ignorant, barbarous, thoughtless, irreclaimable, unsociable foreigners of the Orc islands."
Mothers, Ravens and Annals
The Orkneyinga Saga, which claims that Sigurðr's mother was Irish, tells us he died holding a raven banner given to him by his mother. It had been foretold that this banner would bring life to those who fought beneath it, but death to the man who bore it. Thus, Sigurðr was killed in battle shortly after he took hold of the banner.
The Cogadh tells us that it was Murchad, Brian Boru's son, who slayed Sigurðr.
It was then that Murchadh perceived Siurcaid, son of Lotar, Earl of Insi Orc, in the midst of the battalion of the Dál Cais, slaughtering and mutilating them; and his fury among them was that of a robber upon a plain; and neither pointed nor any kind of edged weapon could harm him; and there was no strength that yielded not, nor thickness that became not thin.
Then Murchadh made a violent rush at him, and dealt him a fierce, powerful, crushing blow from the valiant death-dealing, active right hand, in the direction of his neck, and the fastenings of the foreign hateful helmet that was on his head, so that he cut the buttons, and the fastenings, and the clasps, and the buckles that were fastening the helmet; and he brought the sword of the graceful left hand to hew and maim him after the helmet had fallen backwards from him; and he cut his neck, and felled that brave hero with two tremendous, well-aimed blows, in that manner."
While some of these later sagas may exaggerate the details of the battle, Sigurðr's participation at Clontarf in 1014 is confirmed by the Irish annals, where he is described as a senior commander leading a considerable force.
Theodore M Andersson, 'The King of Iceland' in Speculum Vol. 74, No. 4 (Oct. 01, 1999), pp 923-934.
Alexander Bugge, 'The Origin and Credibility of the Icelandic Saga' in The American Historical Review Vol. 14 No. 2 (Jan 01, 1909), pp 249-261.
Robert Cook, intro. and trans., Njal’s saga (London, 2001).
Lars Lönnroth, Njál’s saga: A Critical Introduction (Berkley,1976).
Magnus Magnusson and Hermann Palsson, intro. and trans., Njál’s saga (Harmondstown,1960).
William Ian Miller, Bloodtaking and Peacemaking: Feud, Law and Society in Saga Iceland (Chicago, 1990).
Harold Mytum, 'The Vikings and Ireland: Ethnicity, Identity, and Culture Change' in James H. Barrett (ed), Contact, Continuity and Collapse: The Norse Colonization of the North Atlantic (Turnhout, 2003), pp 113-137.
Donnchadh Ó Corráin, 'Viking Ireland: Afterthoughts' in Howard B Clarke, Máire Ní Mhaonaigh, and Ragnall O’Floinn (eds), Ireland and Scandinavia in the Early Viking Age (Dublin,1998), pp 421-452.
Brian Ó Cuív, 'Personal Names as an Indicator of Relations Between Native Irish and Settlers in Viking Period' in John Bradley (ed), Settlement and Society in Medieval Ireland: Studies presented to F.X. Martin, O.S.A. (Kilkenny, 1988), pp 79-88.