If you are responding to a question that uses the copula, then you have to use the copula in the answer - An lá fuar é? "is it a cold day?" - Is ea "It is".
If the question doesn't use the copula, and uses the verb bí instead then you use tá sé as the answer - An bhfuil sé fuar? "Is it cold?" - Tá sé "It is"
Notes: an bhfuil is interrogative form of bí in the present tense.
You don't need to include the sé in this case - just tá is sufficient.
(If you don't understand when the copula is used yet, don't worry, it'll get easier with practice, but using the verb bi when you should use the copula is a frequent mistake for beginners.
'S ea or an ea with the copula, tá with bí. That's clear. But what about with other verbs? I would welcome any clarification. For example. "An léann tú ar maidin? Léim." Is it legitimate, instead of léim to answer with 'S ea? I'm guessing not, but it would be good to be clear on this point.