I don't think that's the case (minch= whitecaps).

The Gaelic names for the Minch (both parts) are completely unrelated to the word, and it's used in several places in the UK to mean a somewhat crooked channel or body of water.

From my (admittedly fairly basic, but including a lot of dialect, historic and seafaring terms, since I worked on Norwegian vessels for several years in my youth) Norwegian, it doesn't seem to fit with any old Norsk, Ny Norsk or Bøkmal Norsk terms or word roots that I know, so I'd be very much inclined to think that the derivation is the same as "Manche" as used in French for the Channel.

Island names such as Mingulay are certainly of scandinavian origin. They match the Norwegian ending "øy" for island and there are several possibilites for the fore-part of the name. Probably the most logical would be the old Norsk for "big"...ie "Big Island" since, although Mingulay isn't actually the biggest of its local group, standing tall it *looks* to be the biggest from a boat. Giving descriptive names ("sheep island", puffin island", "bird island", etc) seems to have been very much a habit of scandianvian seafarers in the dark ages.