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There is extensive documentary and archeological evidence for Celtic chain mail; one Classical writer (Varro) even specifically named the Celts as the inventors of chain mail.
You can take a look at this (PDF) article, for example, which includes quotations from Classical authors, contemporary depictions from archeological relics of Celtic warriors wearing chain mail, and fragments of chain mail recovered from Celtic graves dating back to the 3rd Century BC (especially in Eastern Europe and Asia Minor.)