how are adjectives beginning with vowels affected?

why do some plural adjectives get lenitated?

How do the adjectives change in different cases(genitive, dative)?

why do some adjectives such as "sean" come before the noun?

http://nualeargais.ie/gnag/adjekt1.htm



their 1st letter doesn't change.



they get lenited after a nominative or dative plural noun that end with a slender consonant (exception: caoirigh).



in the dative they are as in the nominative.
In the genitive plural they are as in the singular nominative if the noun itself has the same form as its singular nominative. If the noun has another form than its nominative singular (eg. if you add -annaī, -acha, -a, -aī etc), the adjective that follows it takes the same form as in the nominative plural.
Eg.

tithe na bhfear beag
toithe na ndaoinī maithe, na ndaoinī beaga...



it has always been like that in the history of the language, and in Common Celtic. A few adjectives are prefixed. You even find the same ones in Welsh, Breton etc (sean- is hen- in Welsh). Droch- is drwg- in Welsh, and so on. They are prefixed adjectives. But there are very few of them anyway.

I'll leave the strict grammatical explanations to the experts, but here is what I'd say -

"the big green square" An chearnóg mhór ghlas Both mór and glas are lenited due to "cearnóg" being feminine.

""the cats are small, nice and fat" Tá na cait beag, deas agus ramhar (No changes here)

Wait for more input to be sure.