Gallowglass or Gallochglaich(Gaelic name) as they were known over here were warriors primarily from the Hebrides islands and were of Viking descent.They hired themselves out as mercenaries in Scotland and Ireland using their Claymores or Claimh Mor(Gaelic for big sword) and Sparth(big f**k off battle axes).The translation of Gallochglaich means Foreign Warrior.They were not actually an Irish warrior after all but eventually large groups of them were used by Irish chieftains.
Anyway this was all after the year 1000 A.D and well outside the timeline of BI.