You'd have think this would be somewhere on the infinite expanses of the internet, but...what are the vocative forms of tú and sibh? I realize you'd rarely say thú (or is it tú?) amadáin, but how would you say "Hey you" (singular or plural)?

2nd person pronouns are always used to address people.
So, if you will, 2nd person pronouns are the vocative forms of 3rd person pronouns.

On their own, you use emphatic forms: Tusa! and Sibhse!
Haigh, tusa! = Hey, you!

OK, thanks.

On Teanglann, there's also - You there! - Tusa ansin!
They also say -
2 Form thusa used as the object of a verb and in lieu of emphatic suffix with imperative verb :
Thréig sí thusa ar fhear eile, she deserted you for another man. Gabh thusa abhaile, you go home.

Yes, I already knew all of that, thank you.