Hi, I was curious what Irish was here, and this caught my eye
It might be better as part of a question/answer pair.

It's been a while but this point was constantly made at school.
(if I remember correctly) strictly speaking this is closer to "it is" and is a contraction of "is ea". It isn't always an appropriate answer. For example, An tháinig tú? thåinig is the appropriate response not sea.
http://www.daltai.com/grammar/notes/note2.htm

Technically, Irish has a "copula". "Is ea" (contracted to "sea") is an example of its use.
When a question is asked using a copula, the reply should also use it. When the question uses a verb, then it's answered with the same verb.
"Tá sé lasmuigh anois. Nach ea?" -> "He's outside now. Isn't that the case/Isn't that so?"
"Is ea/Sea" -> "Yes [that's right]"
"Ní hea" -> "No [that's not right]"

"Tá" is a verb ("to be", with the dictionary form "bí") so if you ask a question with it as the main verb, you have to reply in kind.

"An bhfuil sé lasmuigh anois?" "Is he outside now?"
"Tá sé" -> "He is"
"Níl se" .> "He's not"

@shiawase: it's "_ar_ tháinig tú"

I've added 'It is.' as an alternative English translation.