This is a Viking boatshed, as found on Shetland and other locations on Scotland's windswept and treeless coast. Boats could be sheltered from the stormy blast but easily carried down the beach when needed.
It is called a noust (or naust, nowst, noost, or noast, depending to the taste and fancy of the speller).
This is great architecture. Simple and practical, but the horseshoe shape adding a touch of elegance.
The Tyree Maritime Trust has created a new boat shed for the trust's small fleet of historic 16ft luggers, and they have called it The Noust. It's good architecture too, more comfortable and a lot warmer but perhaps less of a piece with the landscape.
Thanks to CRABlakeney for the heads-up.