Is the phrase "Tá go maith!" used as reply to a question that's similar in usage to the American English reply of "Sure!" or "Okay!"?

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Just starting my study with Learning Irish by Mícheál Ó Siadhail.

I think so, yes, but I'm more familiar with "ceart go leór" in this meaning.
Tá go maith can mean "very well then", which is a meaning very similar to "OK".

This is helpful. Thank you!

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Just starting my study with Learning Irish by Mícheál Ó Siadhail.

As djwebb said, ceart go leor, literally meaning "right enough" can be generally used for "okay" or "alright". Tá go maith you need to be a bit careful with. In Irish you (generally) have to answer questions using the same verb that was used to ask the question. Tá wouldn't be an appropriate answer in all situations.

I suspect you encountered this in an example in Buntús Cainte. If you'd like to share some portion of the dialogue before this reponse I'd be happy to go into more detail.

ceart go leór has a long vowel, and the vowel is marked long in Muskerry literature. This is not the traditional orthography, but then neither is the traditional orthography the Standard today - and I'm sure Ade (who wrote a couple of days ago on the importance of learning Standardised Irish) will support many non-traditional spellings. Leór it is. There is no word in the Irish language pronounced leor (unless there is some dialect that has a short vowel there).

Nevertheless, if you want to find the word in modern Irish dictionaries like Dineen and Ó Dónaill, you'll find its headword is spelled without the mark of length, leor.

Those dictionaries don't give the pronunciation. Dinneen's is the real language of course. Ó Dónaill's dictionary isn't. Ó Dónaill himself did not use the forms in his own dialect that he peddled in that dictionary.

Thank you for the help!

This came from the first lesson in Learning Irish by Ó Siadhail. He defines Tá go maith as "alright." This phrase was introduced within a list of a dozen or so other phrases with no dialog or context.

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Just starting my study with Learning Irish by Mícheál Ó Siadhail.

It's not a book I'm personally familiar with (it's relatively recent I think). But, it seems to be fairly widely adopted. Perhaps as you get further into it phrases like these will start to be used in the context of a dialogue. In any case, it's not a bad one to commit to memory early on.

All the best with your endeavour to learn the language!

I guess 1988 (or 1980, depending on source, can’t really find good info on the first edition ever) can be called recent relative to stuff like O’Nolan’s Studies in Modern Irish or PUL’s books, or the 1961 TYI… :P

Anyway, although I’ve never been focusing on Connacht or Conamara/Cois Fharraige Irish specifically, I still think it’s one of the best beginner textbooks out there – at least the grammar-heavy ones – and I do reference it sometimes. It’s kinda similar to Polish An Ghaeilge in that it focuses on grammar explanations, gives phonemic transcription to vocab, has a focus on particular dialect, etc. It just teaches Cois Fharraige instead of West Munster Irish.