If Edward I. was, as he himself said, the hammer of the Scots, Somerled was certainly the hammer of the Norsemen. Justice has hardly yet been done to the great work he did in putting an end to the encroachments of the Norsemen on the mainland of Scotland, and in expelling them from Loch-aber and Argyll, and secondly in making the conquest of the isles of Arran and Bute by David, who followed his lead, permanent and successful. His alliance with the daughter of the Norse leader Olave, king of the Isles, was another instance of the statesmanlike policy of the greatest of the old Highland chiefs. He alone it was who made it possible for the later Scottish kings to obtain a foothold in the west, where danger had threatened for so many centuries from the overwhelming sea power of Norway. His conquests made in Argyll, on the mainland, far more than the desultory victories of the Scottish kings, made a united Scotland possible. The help afforded to Bruce by Somerled's grandson, Angus Oig of Kintyre, in the darkest hour of his fortunes, again, made it possible for that king, with a fuller knowledge and wider perspective than were possible to Somerled, to build permanently upon these great beginnings. As the authors of The History of the Clan Donald say—

"Somerled was more than a warrior. He possessed not only the courage and dash which are associated with the Celtic character; he had the organising brain, the fertile resource, the art not only of winning battles, but of turning them to account; that sovereign faculty of commanding the respect and allegiance of men which marks the true king. Without the possession of this imperial capacity he could never, in the face of such tremendous odds, have wrested the sovereignty of the Gael from his hereditary foes, and handed it to the Clan Cholla to be their inheritance for hundreds of years. He was the instrument by which the position, the power, the language of the Gael were saved from being overwhelmed by Teutonic influence, and Celtic culture and tradition received a new lease of life. He founded a family which played no ignoble part in Scottish history. If our faith in the principle of heredity is sometimes shaken by degenerate sons of noble sires, when the last links of a line of long ago prove unworthy heirs of a great past, our faith is confirmed in it by the line of Princes that sat upon the Island throne, who as a race were stamped with the heroic qualities which characterised the son of Gillibride. Somerled's life struggle had been with the power of the Norseman, whose sun in the Isles he saw on the eve of setting. But he met his tragic fate in conflict with another and more formidable set of forces. This was the contest which Somerled bequeathed as a legacy to his successors. It was theirs to be the leading spirits in the resistance of the Gaelic race, language, and social life, to the new and advancing order which was already moulding into an organic unity the various nationalities of Scotland— the ever-increasing, ever-extending power of feudal institutions."

According to Hugh Macdonald's MS. Somerled was a "well-tempered man, in body shapely, of a fair piercing eye, of middle stature, and of quick discernment." His leadership was entirely successful, and his victory was, as Gregory puts it, "the beginning of the ruin of the Norse Kingdom of the Isles." The Gaels from all parts crowded to his banner, and he wrested Argyll and Lochaber from the grip of the Norsemen. And there, in this land of grey hills and green waters, he "made a realm and reigned."

In the Book of Ballymote Somhairle's pedigree is given as "Somerled, son of Gille-brigde, son of Gilliadamnain, son of Solaimh, son of Imergi." But the Book of Clanranald takes us back several steps further. It gives "Somerled, son of Giollabride, son of Giollia-damnain, son of Solomh, son of Mearghach or Imergi, son of Suibhne, son of Niallghus, son of Gothfruigh, son of Fearghus, of the reign of Kenneth MacAlpin."

There can be no doubt that the success of Somerled in clearing the Norsemen out of "the western side of Alban, except the islands of the Finlochlann, called Innisgall," as the Book of Clanranald puts it, relieved the anxieties of the Scottish King David, who was unable to cope with the great power of the Norwegians on his coasts, and doubtless felt that their encroachments on the mainland were a still more serious menace to his kingdom. He, however, took heart and followed Somerled's victory by capturing from the Norsemen the islands of Arran and Bute in 1135, some two years later. These he conferred upon the victorious Somhairle, and allowed him to annex them to the "Kingdom of Argyll," of which he was, it is generally admitted, the hereditary king or chief. By this statesmanlike policy of David the kingdom of the Southern Hebrides became a kind of buffer state between the kingdom of Alban and the Norse Vikings of the Northern or Outer Hebrides and Orkney. It also healed the old quarrel between the descendants of Malcolm, who had made alliance with Magnus Barefoot, that arch enemy of the Innse Gall, and at the same time split any minor alliances that might have existed between the Gaels and the Norse Vikings.

Somerled further strengthened his hold on the Isles, about the year 1140, by marrying Ragnhilda, the daughter of Olave the Red, and sister of Godred, whose harsh and oppressive rule had been the cause of the widespread revolt in which Somhairle had found his great opportunity. Godred soon saw that his enemy was like to crush him out of the rest of the Hebrides, for Somerled in 1156 joined with Thorfinn, a Manx chief, in a plot to place Dugall, a mere child, son of Somerled and Ragnhilda, on the throne of the Isles. Godred heard of the plot, and sailed from Man with a fleet to meet Somerled, who with eighty ships was waiting for him. A terrific battle took place, which, at the end of a long day, found the combatants still determined and unbeaten. Having tasted the quality of his great rival, Godred was glad to make terms by which all the islands south of the point of Ardnamurchan, of course including Kintyre, always regarded as an island, were ceded to Somerled, or rather to his son Dugall; while Godred kept for himself Man, Skye, Coll, Tiree, and the Long Island. Mr. Dugald Mitchell suggests that a probable result of this arrangement was an exodus of the purely Norse population from the south islands, and of the purely Keltic portion of the population of the northern islands, which remained under Norwegian rule till a hundred years later. In 1156 there seems to have been another quarrel between Godred and Somerled, who invaded Man with his fleet and added it to his dominions. Godred fled to Norway, where he remained till Somerled's death eight years later. In 1159 the peace was made between the King of Scotland and Somerled, which resulted in the drawing up of the famous treaty of that date, held to be of so much importance that it formed an epoch for the dating of Scottish charters.

In 1164 Somerled had again fallen out with the Scottish monarch, for whose kingdom he had done so much. His object was, it is said, to make himself king of all Scotland. Be this as it may, he sailed up the Clyde with a fleet of one hundred and sixty ships and a force of Scots from Ireland. He landed, according to tradition, at Renfrew, and Gregory thinks the old story is correct which states that he was there murdered in his tent by one of his own followers in whom he placed confidence. His son Gillecallum was also slain, and his men returned to the Isles. His body was taken to Saddell, in Kintyre, where his son Ragnald built the monastery of which the remains still stand, and endowed it with lands at Boltefean, in Kintyre, and Shisken in Arran.

So died the man who preserved the identity of the Gaels in the western Highlands and the islands of Innse Gall, and who put a stop for ever to the encroachments of the Norwegians on the mainland of Scotland. For, though there were subsequent attacks till the time of Alexander III., no acre of Scottish ground ever again knew a Norwegian owner, and no foothold of any permanence was again obtained even amongst the islands. Only the Orkneys and Shetlands, which never had at any time belonged to the Scottish kings, remained under Norse rule till their cession to Scotland in 1564.