"There are brilliant cats in the Isle of Man."
Translation:Tha cait sgoinneil ann an Eilean Mhanainn.
It seems that when there is no article with the noun (e.g cait), tha = there is/are, but when the noun has a definite article, then tha = is/are. Tha cait sgoinneil ann an Eilean Mhanainn (there are brilliant cats ...) vs Tha na cait sgoinneil ann an Eilean Mhanainn (the cats are brilliant ...). But I have to agree, I'm not entirely sure either why "cats are brilliant" doesn't work. This particular example is a bit clunky in English, there are other more contentious ones - not that I can think of any just at the minute.