"You have a mouse, is that so? No."
Translation:Tá luch agat, an ea? Ní hea.
Just "it is", when responding to a question that uses the copula:
"Is that for me? It is" - An domsa é sin? Is ea
"Is that a mouse? It isn't" - An luch é sin? Ní hea.
But if you are not using the copula to form the question, then you don't use is ea:
"is it still raining? It is" - An bhfuil sé ag cur báistí fós? - Tá
"Is this seat free? It isn't" - An bhfuil an suíochán seo saor? Níl