The first written source for the origin of Clan MacIntyre is the Manuscript History of the MacDonalds handwritten under the nom de plume, Sleat Seanachie c.1647. The story of interest to Clan MacIntyre is near the beginning of the manuscript.
To understand the story, you need to know the players. In this story you have Somerled, Thane of Argyll; Olav II, King of Man; and Muriach MacNeil, a shipwright who is said to be both Somerled's nephew and Olav's foster brother.
In 1140, Olav II, with Muriach at his side, set out with a large fleet of galleys and warriors to re-establish his control of the Western Isles. He stopped to rest and resupply on the coast of Argyll at the by in Loch Sunart, near Somerled's stronghold at Moidart.
Somerled offered to help Olav if the could marry Olav's daughter, Ragnhild. Olav refused and demanded that Somerled sail with him anyway. Outnumbered, Somerled had no choice but to comply. As a courtesy or perhaps to spy, Olav allowed Muriach to transfer to his Uncle Somerled's galley. It was then that Muriach gave his uncle a plan to gain Ragnhild's hand in marriage.
That night the galleys were beached with their bows on land and sterns in shallow water. Everyone was on shore feasting when Muriach secretly waded into the water. He bored holes in the stern and plugged them with tallow and wax. The next morning, when Olav's galley was pushed into the water, the weight of the crew caused the wax-filled holes to go below the water line and out of sight. Olav didn't trust Somerled and had their galleys set side-by-side. After passing the Point of Ardnamurchan, the rough ocean waters dislodged the wax plugs and, as Muriach had planned, water rushed into Olav's galley. Olav called to Somerled for help and he agreed, but only if he could marry his daughter, Ragnhild. Under these dire straits, Olav had no choice but to give his solemn pledge. Once Olav was safely aboard Somerled's galley, Muriach jumped into the cold waters and plugged the holes with wooden pegs he had made, thus saving Olav's galley and crew.
In recognition of his cunning and bravery, Muriach was called, The Wright, or Saoir and from that time forth his descendants have been called Mac an t-Saoir, Sons of The Wright.