Even after reviewing how numbers work, I'm completely not seeing the "one" in twenty-one. Is this not "there are twenty horses"?
Looking at the example at https://www.duolingo.com/skill/ga/Numbers, "21 cats" is"aon chat is fiche" (one and twenty cats). Is this a mistake, or is there another capall hiding out somewhere? Should this not be, "Tá capall aon is fiche..."?
Not really. Remember, that the number usually comes before the noun. However, aon chat is fiche sounds weird. Gramadach na Gaeilge gives it the same structure that's here, and it's all I've seen from native speakers (fáilte is fiche, for example). Basically, what you're saying here is "a horse and twenty".
Ah, OK, got it. It's the "is" that is the clue there. If I wanted to say "20 horses," I wouldn't need the "is" then.
Yes. This is is the abbreviation for agus. And, you're correct - you wouldn't need the is. However, capall fiche is incorrect. You need the number before the noun (except for one, where it'd be capall amháin), so you'd get fiche capall
My compromise learning irish.Sentences like this makes me doubt the structure of this language.