"This is a sheep."
For the moment, that is until we learn about the genitive, it is simple:
- caora singular
- caoraich plural
Of course the problem is not with the Gaelic but with the English, since we use the same word, sheep for both singular and plural. That means, for example, that if you looked these two words up on Google translate, you would get the same English translation.
This may look like a very odd way to form a plural, but that is because it was (apparently) originally more like caorach in the singular. (See Proto-Celtic kaɸrāxs where the /x/ represents a ch sound.) The plural is nothing more than the same sort of vowel change we get in mouse > mice which we will learn later are quite common in Gaelic. For some reason the ch was lost from the singular.
Edit: I should have checked before posting. If you actually do this it finds a really rare second meaning for caora. Bizarre. However if you put the article in it works OK.