I get a little obsessive about who really does what in scenes that look more intimate than comfortable, so I went through interviews, credits, and a few reputable fan interviews to see if there’s any clear confirmation about Fiona O’Shaughnessy using a body double. Short version up front: I couldn’t find a definitive, on-the-record statement from her or the production explicitly saying a body double was used for any intimate scenes. That usually means either there wasn’t one, or the production didn’t publicize it — both totally common outcomes.
From my experience poking around actor interviews and DVD extras, when a double is used it’s often mentioned in behind-the-scenes or in the credits (sometimes as "body double" or "intimacy double") or brought up in press if it was part of a controversy or a talking point about the actor’s comfort and consent on set. Modern productions also frequently work with intimacy coordinators and careful camera blocking, which can make a scene look very real without a double. I kept an eye out for mentions tied to Fiona’s work in projects like 'The Hallow' and 'The Fall', but nothing reliable came up about doubles for intimate scenes.
I tend to give productions the benefit of the doubt when there’s no public info: either sensible precautions were taken quietly, or the scene didn’t require a double. Whatever the case, it’s a good sign that nothing scandalous leaked — in my book that usually means the set handled it professionally. I like that movies and TV are getting better at protecting performers, and that thought makes me respect the craft even more.