Somairle first appears on record in the 1150s, when he is stated to have supported the cause of Máel Coluim mac Alasdair in a rebellion against Malcolm IV, King of Scotland. Máel Coluim was a member of a rival branch of the Scottish royal family, and his sons were closely related to Somairle. At about the time of the rebellion's collapse, Somairle appears to have shifted his energies from Scotland towards the Isles. In 1156, he wrested about half of the Kingdom of the Isles from his brother-in-law, Guðrøðr Óláfsson, King of the Isles. Two years later, Somairle decisively defeated Guðrøðr, gaining complete control of the kingdom.