re: I have several questions: Who is/was Terry O'Sullivan? When did he write the song? Has it been recorded?
...this is a very late reply, and not too informative...

I was looking for where the Acushla Mine words came from and came across this. I have the book that's credited at the end of the lyrics (Songs and Dances of Ireland
published by Wise Publications, 1982)
and it does indeed have the exact words, but it doesn't credit Terry O'Sullivan, or anyone. It just says Traditional, though I guess this is referring to the music rather than the lyrics. At the foot of each song it states the Copyright holder: in this case "Copyright 1982 Dorsey Brothers Music Ltd, London". So it appears that it was copyrighted the same year as the book of songs was published. It isn't clear if the copyright is for or includes the lyrics or is simply the arrangement, but it seems strange to me not to credit the writer if it was done for this book... other songs in the book do state "words by....". So on the whole, I suspect the words are traditional, though their web presence is very slight indeed. So the question then is where does the reference to Terry O'Sullivan come in?
I may well be wrong and he did write the words - I did try to look up Terry O'Sullivan, without success. Songwriter Gilbert O'Sullivan has/had a brother Terry who looked after his affairs in the USA and sued Biz Markie - https://www.nytimes.com/1991/11/29/nyregion/composer-who-knows-he-s-heard-that-song-before-sues.html - But no reason to think it's him, and clearly Terry O'Sullivan is a very common name. But if you're out there, please say!