I know, I know, I'm connecting history to a video game [Ryse: Son of Rome (2/10 wouldn't recommend)]. But curiosity got the better of me. Are there really men that make average barbarians look like civilized men with how ruthless they are? Dressing themselves completely in deer bones so anyone of that time Era would think they were monsters and not men? Here's a link to a part of the game that shows it: https://youtu.be/k-e1MQnh3V0
Here's a link to part a nordic movie, "the 13th warrior" (at 6:50) which pulls the same idea but with nords dressed in bear pelts to look like beasts: https://youtu.be/1PiyklMmrTQ
By the time you are that far north "celtic" is really rather questionable term. Iron age Britons would probably be more accurate.
We really don't know that much about the area and its more proto-history than history. We know the romans called some of the groups and there is archaeological evidence. The groups in southern appear to have been fairly typical Iron age Britons just with less romanisation than southern groups. The north of scotland we know even less about. Other than Gnaeus Julius Agricola militry involvement in the area we have few written records. There does appear to have been a couple of seperate cultures in the area with little souther celtic infludence and later only limited roman infludence but other than fortifications declining over the period of roman rule there aren't many patturns. They also appear too have had less acess to iron than more southern groups.
Source:Barry Cunliffe's, Iron Age Communities in Britain: An Account of England, Scotland and Wales from the Seventh Century BC until the Roman Conquest 4th ed.