Last time looking at this I came across some court records concerning a Thomas Gregory and trespass charges.
The charges were what looks like (to me)
"His goat (?) trespassing on compts (complainants) land at Killester north at Clontarf "
5 of them in all between 13th Dec and 7th Janurary 1873/74.
The complainant was a Thomas Carolyn (initially couldn't make out the name) but I have now established that he had numerous lease's to land's in Clontarf, Killester and Raheny. (Strandville, Hollybrook, Strand Road) The Carolan family home was St Edmunds and they also owned the pub on the Howth Road, now named Harry Byrnes)
St Edmund's is now The Yacht pub and apartment building on the corner of Seaview Avenue to the left.
This map will probably explain better.
http://maps.osi.ie/publicviewer/#V1,718846,736102,7,9(Seaview Ave is in the center at the red cross - you can toggle between it and Google)
The trespassing Goat could definitely have been wandering up north from Seaview Avenue, Clontarf to the wooded land shown in Killester
Can't be positive it's your Thomas Gregory but suggesting that any search for death of Thomas Gregory might concentrate first between 1874 and the 1880 marriage cert of his daughter which lists him as dead.
I really can't understand the total lack of newspaper records of the initial mine accident and most especially the 1980's discovery. Have you any other detail at all
Certainly the 1980's discovery would have entailed the involvement of the coroners office, city council archeology, reburial and I'd imagine a lot of media, how could they have identified them ?
So am really, totally stumped there