Somerled was slain in 1164 at the Battle of Renfrew, amidst an invasion of mainland Scotland, commanding forces drawn from all over his kingdom. The reasons for his attack are unknown. He may have wished to nullify Scottish encroachment, but the scale of his venture suggests that he nursed greater ambitions. On his death, Somerled's vast kingdom disintegrated, although his sons retained much of the southern Hebridean portion. Compared to his immediate descendants, who associated themselves with reformed religious orders, Somerled may have been something of a religious traditionalist. In the last year of his life, he attempted to persuade the head of the Columban monastic community, Flaithbertach Ua Brolcháin, Abbot of Derry, to relocate from Ireland to Iona, a sacred island within Somerled's sphere of influence. Unfortunately for Somerled, his demise denied him the ecclesiastical reunification he sought, and decades later his descendants oversaw the obliteration of the island's Columban monastery. Iona's oldest surviving building, St Oran's Chapel, dates to the mid-12th century, and may have been built by Somerled or his family.
Traditionally imagined as a Celtic hero, who vanquished Viking foes and fostered a Gaelic renaissance, contemporary sources instead reveal that Somerled operated in, and belonged to, the same Norse-Gaelic cultural environment as his maritime neighbours. By his wife, Ragnhild, daughter of Olafr Godredsson, King of the Isles, a member of the Crovan dynasty, Somerled and his descendants claimed the Kingdom of the Isles. A later medieval successor to this kingdom, the Lordship of the Isles, was ruled by Somerled's descendants until the late 15th century. Regarded as a significant figure in 12th-century Scottish and Manx history, Somerled is proudly proclaimed as a patrilineal ancestor by several Scottish clans. Recent genetic studies suggest that Somerled has hundreds of thousands of patrilineal descendants, and that his patrilineal origins may lie in Scandinavia.
Somerled's career is patchily documented in four main contemporary sources: the Chronicle of Holyrood, the Chronicle of Melrose, the Chronicles of Mann, and the Carmen de Morte Sumerledi. The chronicles of Holyrood and Melrose were originally compiled in the late 12th century. As products of Scottish reformed monasteries, these sources tend to be sympathetic to the cause of the Scottish kings descended from Malcolm III of Scotland. The Chronicle of Mann was first compiled in the mid-13th century, and concerns itself with the history of the Crovan dynasty, a rival kindred of Somerled and his descendants. For similar reasons, the aforementioned sources and the Carmen de Morte Sumerledi, a late 12th-century Latin poem by a Scottish cleric who witnessed Somerled's final invasion against the Scots, are partisan accounts slanted against Somerled. Various Irish annals are also useful sources of information, although they usually only corroborate what is documented in other sources. Later clan histories, such as the early modern History of the MacDonalds and the Books of Clanranald, although unreliable as historical narratives, contain a considerable amount of detailed information. The late provenance and partisan nature of these histories means that their uncorroborated claims, particularly those concerning early figures such as Somerled and his contemporaries, need to be treated with caution. Another relevant source is a particular charter, issued by Malcolm IV, King of Scotland (d. 1165) in 1160, that briefly notes Somerled in its dating clause.
Somerled's origins are masked in obscurity and myth. Although no contemporary pedigree exists that outlines his ancestry, there are over a dozen later medieval, early modern, and modern sources that purport to outline Somerled's patrilineal descent. The names that these sources give for his father (GilleBride) and paternal grandfather (GilleAdamnan) appear to be corroborated in patronymic forms recorded in the Annals of Tigernach and the Annals of Ulster. The names in preceding generations, however, become more unusual, and the more authoritative sources begin to contradict each other. In consequence, two or three generations may be the furthest that Somerled's patrilineal lineage can be traced with any degree of accuracy. Somerled was almost certainly of Norse–Gaelic ancestry, and nothing is known of his early life. The History of the MacDonalds and the Book of Clanranald relate that his immediate ancestors were prominent in Argyll before being unjustly ejected by Scandinavians and Scots. Although these specific claims concerning his ancestors cannot be corroborated, Somerled's eventual marriage to a daughter of a reigning King of the Isles, and the marriage of one of the former's immediate kinswomen to the son of a King of Scotland, suggests that Somerled belonged to a family of considerable status.
The precise identity of Somerled's aforementioned kinswoman is uncertain. The following pedigrees illustrate three possible ways in which her marriage bound Somerled's family with a senior branch of the Scottish dynasty. According to the Chronicle of Holyrood, the sons of Máel Coluim mac Alaxandair (fl. 1134), son of Alexander I of Scotland (d. 1124), were Somerled's "nepotes". This Latin term could be evidence that the mother of Malcolm's sons was either a sister, or a daughter of Somerled; or Somerled and Malcolm were maternal half-brothers.