"She is a beautiful girl."
Translation:Is cailín álainn í.
I am not quite sure now, but I think aláinn is an attributive adjective here and cailín is a maskuline noun. Wouldn't I have to put 'h' before aláinn? Or is it only with a feminine noun that the adjective gets an 'h' before it? Or did I mix up the rules how to change adjectives completely?
Attributive adjectives are lenited after feminine nouns (in the nominative).
Words that start with a vowel can't be lenited, and a h- prefix isn't lenition, so attributive adjectives that start with a vowel don't get a h-prefix whether the noun is masculine or feminine.
Adjectives that start with a vowel only get a h- prefix after go - go hálainn, go háirithe, go hiomlán, and after chomh - chomh híseal, chomh hóg, chomh hamaideach