Is there a difference on when to use sibh vs. tú?
In modern standard irish, sibh for plural you (ye, yous) and tú for singular. It doesn't have the formal/informal use like german or french.
I guess for examples which don't really have context/aren't clear, either of them should do.
When would you use stobhach or stobhaigh?
stobhaigh is the genitive of stobhach, so it would be used when you are using "stew" as an attribute, though that's usually "stewing", as in "stewing beef" - mairteoil stobhaigh, and it can also be used to say "a bowl of stew" - babhla stobhaigh.