Not only that, it's also bad Gaelic. I'm almost 100% convinced this is made up and not historic. I think the only time I've heard the Shetland mainland spoken about in Gaelic was on BBC Alba as Tìr-mòr Shealtainn but again that's not historic as such and just a literal translation of Shetland Mainland. I think until we can establish at least modern usage via BBC Alba, we should go without a Gaelic name.