I usually believed that chain mail was indeed developped by Celts who were
master ironworkers, but your question raises the point that mail would have
been developped in response to the type of weaponry that the wearer would
be facing. Was mail developped because it was more efficient against
arrows? Arrows were not very well known or used in the Celtic world until the
beginnings of the Roman conquest, but they certainly were in the Middle East.
If not, would mail have been developped against the big Celtic and Germanic
slashing swords? It sounds possible.