Gallowglass is a word that I learned in early childhood as it is mentioned in 'Clan Connell War Song' (aka 'O'Donnell Abu'). The tune was popular in the 40s and 50s as a signature tune on Radio Eireann. The Gallowglass were mercenary soldiers in Ireland from the Scottish Western Islands and Argyll.

It is defined in the Dictionary Of Hiberno-English as,

'galloglas, noun, a mercenary of Scottish or Hiberno-Norse origin; a heavily armed soldier in a chief's retinue. From the Irish 'gallóglach; gall = foreigner, óglach = army.

There's a well known ceili band called Gallowglass.

(sorry I don't know anything about the Auchindoun song!)