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.

There is an implied copulation with the cé which serves as the verb. The answer would be Is liomsa or Ní liomsa assuming you owned or didn't own the tra.