Historically, the primary mail garment used by a knight was a simple shirt, called a byrnie or birnie. The flexible short sleeved, waist length shirt was made of interlocking metal rings that protected from slashing attacks, and was the precursor to the haubererk. The haubererk was often rather long, falling well past the waist and sometimes as far as the knees. In old Irish “ birnie” was defined as the “knight in shinning armour”.  See Nordic Origins ? for insight.

    As a traditional story, it is written that in 838 A.D., while serving as a Knight to Kenneth MacAlpine (Scotland's first King), Birnie (sp.MacBirnie) and his two sons were captured by the Picts. Birnie, as a descendent of the Dalradians, was facing an automatic death penalty. The three escaped the stocks by cutting off their lower legs. As a reward for bravery in the war that united Scotland, Birnie was bestowed the title of Baron, and was awarded the lands south of Elgin. For Dalradian’s history see Wikpedia.

  Geographically, the area was a moist, oozy place that had abundant hazel nut trees. (Birnie, before 1200 was "Brennack"), simply G. Broanach, a moist place. The dative-locative is "braonaigh", which becomes "birnie" in Scots by the usual metathesis (from History of Celtic Name Places in Scotland by W.J. Watson, 1926).

   Note, that the origins of MacBirnie, a Sept of Clan Matheson, is a West Coast Clan that appears in Western Ross. The Clan reportedly fought with Kenneth Mac Alpine, against the Picts. This coincides with John Birnie’s narrative of a Birnie and his sons, who did fight for the first King of Scotland. (See Lord Hamilton). The Mathesons (Clan of the Bear) later dispersed, often under the MacKenzie Clan.