Fergus forged a marital alliance with Óláfr Guðrøðarson, King of the Isles through the marriage of the latter to Fergus's daughter, Affraic. As a consequence of this union, the leading branch of the Crovan dynasty descended from Fergus. When Óláfr was assassinated by a rival branch of the dynasty, Galloway itself was attacked before Fergus's grandson, Guðrøðr Óláfsson, was able to seize control of Isles. Both Fergus and his grandson appear to have overseen military operations in Ireland, before the latter was overthrown by Somairle mac Gilla Brigte, Lord of Argyll. The fact that there is no record of Fergus lending Guðrøðr support against Somairle could be evidence of a slackening of Fergus's authority. Contemporary sources certainly report that Galloway was wracked by inter-dynastic strife during the decade.