I have a feeling,the celts used chainmail before the romans.The romans then got hold of the idea and expanded on it.
As the materials where hard to work,not many celts could have a full suit but would cover smaller areas such as the upper body.
The romans had a far better organization of resourses so were able to mass produce the chainmail.
I have worn a 15 lb upper suit made to celtic standards for serveral days on end,and once it is on you quickly forget it untill you get hit.
Not much good with a trust from a knife,spear or sword,But great against slashing blows.