"Who owns the tea?"
Translation:Cé leis an tae?
How would we know when to use leis vs. ag? To own something is to be with it in Irish, but to have something is for it to be at you?
It really isn't helpful to think of leis as "with it" in this case, any more than it is to think of ag as "at" in tá tae agam.
Is liomsa an tae can be translated as "the tea is mine" or "I own the tea". Tá an tae agam means "I have the tea". There really isn't any confusion between the two, in either Irish or English.