It's highly counterintuitive, but open fires are actually pretty terrible for keeping your body temperature up. As with most of the northern world, early farmers in the shetlands would have used animal skins for clothing, alongside weather resistant shelter to live in. As the Inuit today can attest, layers and layers of skins will keep you warm in almost any conditions when properly made. Some of these skins would undoubtedly have been traded in with the larger islands, but some would have been produced by local stock.