I would suggest you don't use the stone barn for sheep housing at all. Where we live the weather is also wet and very windy, sometimes with deep snow in winter. When we started with sheep many years ago, we had Jacobs. We were told that they had to lamb indoors, so we kept them in a loosebox in a stone barn. For the couple of years we did this, we had lambing problems and lost a ewe to pneumonia, and a lamb to E Coli. We also had Hebrideans, which never came in - we had no lambing problems and no pneumonia. We made outdoor shelters (you can see pics of one or two of them on our website (below)) and let the sheep choose to take shelter or not (sheep are far from daft and make very sensible decisions if left to their own devices). Since then we have kept a variety of breeds and have kept them outside all year round, including for lambing. For the occasional problem we will bring a ewe and new lambs into a hurdle pen in the polytunnel, with a through-draft, to mother up.
Have you thought of putting up shelters, with three sides and a very well attached roof? They are very simple to build, and can be based around four ordinary fence stobs. leave a gap of an inch or so between the side planks so the wind whistles through - sounds chilly but sheep have thick coats and are fine - it's the wet that gets them
Some breeds suffer more from the wet than others, Hebrideans for example can have a good shake in a heavy rain storm and their fleece appears almost dry; down type sheep with finer, denser coats tend to hold the wet more. This is where the gaps between the slats of the shelter come in, as the sheep can stand out of the rain under a roof, but blow-dry in the wind.
As Anke says, what breed are your sheep, as that will affect their needs where you live?