Here's the entire panel, which is in the Shetland Museum, where it is known as “The Monk Stone”, identified as a Pictish altar with a carving of Christian missionaries.
These guys appear to be wearing a long version of the one-piece hooded garment called in Latin birrus Brittanicus (which I discussed recently in Irish here). The birrus, which was made of wool and was reasonably waterproof as well as warm, was common in Gaul and Britain during the Roman period and into the Middle Ages. The word birrus comes from the Gaulish word birros, meaning “short”. The original version consisted of a hood and short cloak which covered just the shoulders and upper body. (The French word beret derives ultimately from the same word and garment, via Italian berretta.)