Gradually, over time, the knights who wore a Birnie, were referred to as Birnies  (a similar name evolution as 'Smith', or 'Baker'). Also note there are many references to Birnie shirts, and then named Birnies throughout Norwegian literature. The surname of Birnie was retained by knights even as these shirts evolved to include the use of metal breast plates and armour. Remember, this was an age when knights were warriors and mercenaries, not the "knighted" social awards of today. The few that survived earned the Roman tradition of receiving land grants & freedom from military service.

“In the First Helgi-lay, Sigrún rides with a company of battle-maidens, all of them birnie-clad and armed with helmet, spear, and bow"