Without getting into the Celtic debate, I answered a question on warfare in iron age Ireland here that goes into a bit of detail on weaponry, and swords in particular

Unfortunately, because of the organic nature of clothing it almost never survives archaeologically and we can't say how people wore their clothes in the Irish Iron Age with any degree of certainty.

We don't really know much about styles of clothing in much of Bronze Age and Iron Age Europe, but I do have several answers that discuss what we do know of the textiles used:

What (specifics) do we know about clothing of Northern/Central European men during the late Bronze- / early Iron-age?

Did the Bell Beakers (>2300 BC) and Native Britons (<2300 BC) wear armour or advanced clothing (smth. like Östi the Iceman)?

What would Boudicca have worn?