toilet is an taigh-beag with a hyphen, an taigh beag is just the small house¹. And they differ in the stress placement in speech, see this article on the Akerbeltz wiki.
Especially this part:
Sp far, the most obvious example I have come across is the difference in meaning between Tha mi a' fuireach ann an taigh beag and Tha mi a' fuireach ann an taigh-beag. I'd rather not live in the second. In writing Gaelic, you can afford not to pay attention to stress placement and simply learn that the word for toilet has a hyphen. However, when it comes to speaking, you have to know where to place the stress unless you want to make people quietly move away from you when you tell them about your new "home", at the next party you go to.
- taigh-beag with a hyphen, pronounced /təˈbeg/ (taigh reduced to /tə/, stress on beag): a toilet,
- taigh beag without a hyphen, pronounced /tɤj beg/ (with primary stress on fully pronounced taigh): a small house.
EDIT: plural toilets would be taighean-beaga, but I’m not entirely sure how (and if) that would differ in pronunciation from taighean beaga in the meaning small houses.
¹ at least that’s a popular convention of distinguishing them in writing, used also on Duolingo; there are probably some texts with taigh beag without a hyphen in the meaning of toilet – but even if, they’re in the minority