I saw on this website …
… that Julie Fowlis plays the low whistle.
I wasn’t aware of that, but I found this picture …
… so, sure enough.
Does anyone know if any of her album tracks feature her on the low whistle? I’ve checked and don’t see any offhand. Or even a youtube clip.
I know on the Transatlantic Session recordings Michael McGoldrick plays low whistle on a few tracks of hers. Not sure about her actual albums but it wouldn’t be much of a stretch if she played low whistle (pun May or may not be intended) 
Well, at least I’ve found a youtube video with Julie playing low whistle (at 1:27):
She played a low A on the Under One Sky tour a few years ago - IIRC she said on stage that she’d been lent it by John McCusker.