The closest that I’ve found to connect the noun áthas to the adjective “joyful” (other than the adjective áthasach) is the prepositional phrase faoi áthas, such as in Psalms 113:9 —
Tugann sé sliocht sa teach don bhean gan leanbh; go mbíonn ina máthair chlainne faoi áthas.
Thus, I’d expect Tá sibh faoi áthas to be the translation of “You are joyful”, if áthas rather than áthasach is to be used.
It isn't actually true that you usually add a 'h' when vowels meet - tá an is probably one of the most common word pairs in Irish, and the possessive adjective a only adds a h-prefix to words starting with a vowel when a means "her". There are some things in Irish that developed because certain clashing vowels can be awkward to say, but it definitely isn't a general rule.
Resetting this indent because it won't let me continue. It doesn't matter how many people the you is referring you when the family or the team is the focus! Your family, yeer family, your families or yeer families, presumably the person or people I'm talking to is included in their own family. So, taking the unit as a whole it would be "ye did great" for example. You and your family did great, ye and yeer family did great, you and your families did great, ye and yeer families did great. The [you and your family] is the plural that the ye refers to. Yeer is much less commonly written, but it's definitely said. The focus is on the plural unit, and would probably be used in more formal language like I said. This is a question of Hiberno-English grammar, so I'm not sure if it belongs here other than to say that "ye" does indeed exist and is commonly used.
Every single person I know uses ye or yous regularly in both speech and writing, it's not at all uncommon! Plural forms of you eliminate the ambiguity present. If you meant in formal writing, sure, but even then I've found that it's more common to use one, we, or a passive voice, or to refer to the group directly, your team, your family, than to even refer to more than one person