Clan Donnachaidh, which means the children of Duncan, emerged from the union of the Scottish and Pictish kingdoms and counts amongst its forebears the Kindred of St. Columba.
The main clan name and that of the chiefs is Robertson, but many other names taken by those descending from the chiefs or belonging to the families who lived on clan estates and sought the chief's protection can be considered septs of the clan.
Of these, Duncan and Reid are the most prominent. Many others are included and there are many variations on their last name.
Abbot Crinan of Dunkeld (?-1045), descended from the kindred of St. Columba (521-597), was father of Duncan (1004-1040), King of Scots. We know that Duncan was killed by MacBeth (many thanks to William Shakespeare, with his great literary license). Fortunately, Duncan's subsequent descendants held the throne for two and a half centuries more after a couple of convoluted short-term changes in management after MacBeth. Duncan had a younger son, Maelmare, who survived his assasination and became Earl of Atholl. Maelmare was the royal ancestor of the Chiefs of Clan Donnachaidh.
The clan chiefs are numbered from Duncan the Stout (stout in battle rather than in belly) who lived in the 1300s. He held lands in Rannoch and around Glen Errochty and took his followers to fight at Bannockburn in 1314 in support of his friend, King Robert The Bruce. His son Robert (perhaps called after Bruce) inherited land from his own mother and his estate ran from the edge of the Grampians to the gates of Perth.
In 1437 the chief Robert Riach (grizzled) captured Sir Robert Graham who, with others, had just murdered the King James I at Perth. In reward James II gave Robert a charter in which all of his lands were made into a feudal barony giving him administrative control over them.
The barony was called Struan and the chief was henceforth known as Robertson (from this Robert) of Struan
Source: Clan Donnachaidh Website, Clan Donnachaidh Centre, Bruar, Pitlochery
The banner at the top of the page is officially called the "modern Red Robertson" tartan and is made from the "new" chemical dyes which evolved during the moderization of the textile industry during the early-18th century industrial revolution (circa 1725). It has been registed with the Lord Lyon, repository of all ancestral data worldwide. Prior to that time, "natural" dyes were made from roots, berries, flowers, soil and other natural sources.