Clan Donnachaidh also known as Clan Robertson, is one of the oldest of all Scottish clan

Scottish clan

Scottish clans , give a sense of identity and shared descent to people in Scotland and to their relations throughout the world, with a formal structure of Clan Chiefs recognised by the court of the Lord Lyon, King of Arms which acts as an authority concerning matters of heraldry and Coat of Arms...

s.
There are two main theories as to the origins of the Clan Donnachaidh:

1. That the founder of the clan, Donn(a)chadh (Duncan) was the second son of Angus MacDonald
   Aonghas Mór
   Aonghas Mór , also known as Aonghas a Íle and Aonghas mac Domhnaill , was the son of Domhnall mac Raghnaill, eponymous progenitor of Clan Donald.Aonghas Mór has been called "the first MacDonald" by one historian, namely...
   
   , Lord of the Isles
   Lord of the Isles
   The designation Lord of the Isles is today a title of Scottish nobility with historical roots that go back beyond the Kingdom of Scotland. It emerged from a series of hybrid Viking/Gaelic rulers of the west coast and islands of Scotland in the Middle Ages, who wielded sea-power with fleets of...
   
   .
2. That the Robertsons are lineal descendants of the Celtic Earls of Atholl
   Duke of Atholl
   Duke of Atholl, alternatively Duke of Athole, named after Atholl in Scotland, is a title in the Peerage of Scotland held by the head of Clan Murray...
   
   , whose progenitor was King Duncan I
   Duncan I of Scotland
   Donnchad mac Crínáin was king of Scotland from 1034 to 1040...
   
    (Donnchadh in Scottish Gaelic), eldest son of Malcolm II
   Malcolm II of Scotland
   Máel Coluim mac Cináeda , was King of the Scots from 1005 until his death...
   
   .

The clan's first recognised chief, Donnchadh Reamhar, 'Stout Duncan', son of Andrew de Atholia (Latin

Latin

Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 'Andrew of Atholl

Atholl

Atholl or Athole is a large historical division in the Scottish Highlands. Today it forms the northern part of Perth and Kinross, Scotland bordering Marr, Badenoch, Breadalbane, Strathearn, Perth and Lochaber....

'), was a minor land-owner and leader of a kin-group around Dunkeld

Dunkeld

Dunkeld is a small town in Strathtay, Perth and Kinross, Scotland. It is about 15 miles north of Perth on the eastern side of the A9 road into the Scottish Highlands and on the opposite side of the Tay from the Victorian village of Birnam. Dunkeld and Birnam share a railway station, on the...

 , Highland Perthshire

Perthshire

Perthshire, officially the County of Perth , is a registration county in central Scotland. It extends from Strathmore in the east, to the Pass of Drumochter in the north, Rannoch Moor and Ben Lui in the west, and Aberfoyle in the south...

, and as legend has it, an enthusiastic and faithful supporter of Robert I

Robert I of Scotland

Robert I , popularly known as Robert the Bruce , was King of Scots from March 25, 1306, until his death in 1329.His paternal ancestors were of Scoto-Norman heritage , and...

 (king 1306–29) during the Wars of Scottish Independence

Wars of Scottish Independence

The Wars of Scottish Independence were a series of military campaigns fought between the independent Kingdom of Scotland and the Kingdom of England in the late 13th and early 14th centuries....

; he is believed to have looked after King Robert after the Battle of Methven

Battle of Methven

The Battle of Methven took place at Methven in Scotland in 1306, during the Wars of Scottish Independence.-Comyn's Death:In February 1306, Robert Bruce and a small party of his followers killed John Comyn, also known as the Red Comyn, before the high altar of the Greyfriars Church in Dumfries...

 in 1306.