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.

Scotland consolidates authority in the maritime region between the Lennox and Cowal, and along the eastern coast of the Firth of Clyde towards Galloway, in the early 1160s.

David may well have begun the infeftment and settlement of this coastal district decades earlier, in order to counter the sea borne threat that the rulers of Argyll had posed during the dynastic challenges of the 1130s.

Some of the greatest Scottish magnates have taken root in the region by the 1160s, and it is not impossible that some of them may have begun to extend their influence into southern Argyll and the Islands of the Clyde.

The catalyst for Somerled's invasion may therefore be the encroachment of Scottish influence into his own sphere of hegemony.

The target of his invasion appears to have been Renfrew, the center of the family of Walter FitzAlan, Steward of Scotland, and Somerled's forces may well have engaged those of Walter—possibly even led by the steward himself.

When the Anarchy took hold in England and civil war between Empress Matilda and Stephen, Walter FitzAllan, the third son of a Breton knight, had rallied to the support of the Empress.

When her cause was lost, Walter had befriended David I who was an uncle of Matilda, and became David's dapifer or Steward.

Accompanied by his brother Simon, Walter had come to Scotland about 1136 and fought for Scotland at the Battle of the Standard at Northallerton in 1138 under the command of David I's son, Prince Henry.

Subsequently he had been appointed Steward of Scotland by King David I; in 1157 the appointment as Steward had been confirmed as a hereditary office.

In return for the service of five knights, David had also granted him what will eventually comprise Renfrewshire: the lands of Paisley, Pollok, Cathcart, and Ayrshire; this grant had been reconfirmed in a charter in 1157 from Malcolm IV.

The two belligerents, after landing and marching towards Renfrew, meet near Paisley and battle begins.

The Scottish royal army, commanded by the High Steward, Walter FitzAllan, consists of Scoto-Norman knights and armored men-at-arms, and Somerled's Gaelic and Norse warriors are no match against them.

Somerled is wounded in the leg by a javelin and then killed by the sword of his opponents.

Somerled's eldest son Gillecallum, from his first marriage, dies by his side.

With Somerled's death, the Norse-Gaelic army takes flight and many are slain before the survivors escape back to the ships.

Walter FitzAlan had in 1163 founded at Renfrew a house of monks of the Cluniac order drawn from the priory of Much Wenlock, in his native county of Shropshire.

Upon acquired directly from the Crown the Berwickshire estates of Birkenside and Legerwood on the eastern or left bank of the Leader Water, Walter presents to the monks the church of Legerwood, which they will hold from 1164 until the Reformation in 1560.

The monastery will steadily grow and by 1219 become Paisley Abbey.