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MERP describes a very Celtic looking Dunland, and Celts is what I'll be using.
The Rohirrim are called "strawheads" by the Dunlendings and are often said to be like the Saxons. So if the Rohirrim are blond north German-types, I would think of the Dunlendings being pre-Anglo-Saxon Brythonic peoples. So Celts may be fine. I also wonder about Picts. It has been ages since I have read LotR, so I could be wrong, but I think it unlikely that Dunlendings wore much armour.
The Unfinished Tales has a description of the Battle at the Fords of Isen between Rohan and Isengard, and in it it mentions that the Dunlendings are the only ones tall enough to threaten the Rohan shieldwall, but that Rohan still had the advantage as the Dunlendings weren’t armoured due to the Orc armour not fitting them. So unarmoured for the most part would be right