Celts - Weapons and Warfare
Note: Written primary sources for this topic are mostly Roman sources - be aware that the Romans had a very biased approach to describing enemies they believed to be 'uncivilised'. A good idea when working in this area is to compare the written roman sources with the archeological artefacts, especially in Germany and England. You will also find some references to warfare in Celtic poems/sagas as it seems that tribal raids were a part of their culture prior to widespread invasion by Roman armies (with whom some tribes allied themselves against other Celts). I need to try to find the sources for this as my Celtic books were not returned.
One suggestion is to concentrate on a particular period eg Hallstadt or La Tene (most interesting IMO) or the Roman invasion of Britain (easiest). Ireland will be hard because Celtic culture continued there well into the middle ages, so it's hard to tell what's Iron age Celtic culture and what's late middle ages.
From museums
https://artsandculture.google.com/exhibit/celtic-life-in-iron-age-britain/CAKSz1O9u3piJg (click through for the section on warfare)
https://www.youtube.com/watch?v=Hp9psiaoRK8 (has a reconstruction of what may have been a scare-the-enemy device)