Somerled is recorded to have participated in a coup d'état against his brother-in-law, as the Chronicle of Mann relates that Thorfinn Ottarsson, one of the leading men of the Isles, produced Somerled's son, Dugald (d. after 1175), as a replacement to Godred's rule.
As a grandson of Olaf, and the son of a man of with the enterprise and power to confront Mac Lochlainn, Dugald was evidently favored by a significant number of leading Islesmen, disillusioned with Godred's rule; Somerled, therefore, appears to have taken full advantage of the situation in order to secure his eldest son a share in the kingdom.
Be that as it may, Somerled's stratagem does not appear to have received unanimous support, since the chronicle relates that, as Dugald was conducted throughout the Isles, the leading Islesmen were made to render pledges and surrender hostages unto him.
Following an inclusive but bloody sea-battle, possibly fought off Mann in the following January, the chronicle records that Somerled and Godred divided the kingdom between themselves.
According to the History of the MacDonalds, Somerled had previously aided Godred's father in military operations (otherwise unrecorded in contemporary sources) against the "ancient Danes north of Ardnamurchan".
Together with its claim that Olaf had also campaigned on North Uist, this source may be evidence that the partitioning of the Isles between Godred and Somerled can be viewed in the context of Somerled taking back territories that he had helped secure into Olaf's kingdom.
There is reason to suspect that portions of the Isles had previously fallen under the influence of the Earls of Orkney, before being reclaimed by the Kings of Isles during this period.
Following the partitioning, Somerled and Godred appear to have agreed to a truce.
However, about two years later in 1158, the chronicle records that Somerled launched a second assault upon Godred, and drove him from the kingdom altogether.
From this date until his death, Somerled will rule the entire Kingdom of the Isles, and may well have exerted some degree of influence into Galloway.
