Irish doesn't have words for 'yes' and 'no'. What you do instead is repeat the verb in the positive or negative to answer yes or no, which is what's happening here.
Ok, I wrote, "Is it important? Yes, it is." Would that be an acceptable answer?
What about "Is ea."? Would that be acceptable instead of "Tá."? Or is "Is ea." only in response to questions using the copula?
As far as I know, one should answer a question with the same verb that was used in the question. So 'tá' in this case, but 'is ea' where the question also employs the copula.
When we talk about employing the copula, what is meant? I can't make head nor tail of the copula and I have read about it for awhile now. Very much appreciated!
The Tips & Notes for the very first skill, Basics 1 introduce the copula.
Perhaps a simpler explanation is that you use the copula is when you use a noun or pronoun to identify or classify another noun or pronoun - "he is a boy", the noun "boy" is classifying the pronoun "he" - is buachaill é.
This exercise is not using the copula, it is using the verb bí, which is tá in the present tense, or in this cae an bhfuil in the present tense interrogative.