Somerled, known in Middle Irish as Somairle, Somhairle, and Somhairlidh, and in Old Norse as Sumarliði, was a mid-12th-century Norse-Gaelic lord who, through marital alliance and military conquest, rose in prominence to create the Kingdom of Argyll and the Isles. Little is certain of Somerled's origins, although he may have been born in the north of Ireland and appears to have belonged to a Norse–Gaelic family of some prominence. His father, GilleBride, of royal Irish ancestry, appears to have conducted a marriage alliance with Máel Coluim mac Alaxandair, son of Alexander I of Scotland, and claimant to the Scottish throne. During a period of alliance with David I of Scotland, Somerled married Ragnhild, daughter of Óláfr Guðrøðarson, King of Man and the Isles in 1140. In 1153, Olaf of Man died and was succeeded by his son, Godred. But Godred Olafsson was a very unpopular ruler. Somerled was asked by Thorfinn Ottarson, a Manx chief, to allow Somerled's son, Dugall, to be appointed king of Man and the Isles. Somerled agreed and with 80 ships confronted Godred off the coast of Islay on January 5–6, 1156. After the sea battle, Somerled and Godred divided the Kingdom of Man and the Isles between them but Godred did not accept Dugall as King of Man. Accordingly, two years later, Somerled defeated and drove Godred from power. Dugall continued as King of Man and Somerled thus ruled the entire kingdom of Argyll, Man and the Isles until his death.

The Norse–Gaels were a people of mixed Gaelic and Norse ancestry and culture. They emerged in the Viking Age, when Vikings who settled in Ireland and in Scotland became Gaelicised and intermarried with Gaels. The Norse–Gaels dominated much of the Irish Sea and Scottish Sea regions from the 9th to 12th centuries. They founded the Kingdom of the Isles, the Kingdom of Dublin, the Lordship of Galloway, and briefly ruled the Kingdom of York. The most powerful Norse–Gaelic dynasty were the Uí Ímair or House of Ivar.