This is near Annadorn, in the county Down. It was not known that there was any cromleac under this carn, until it was accidentally discovered by a man who was feeding cows beside it.
The cromleac is broad and long, but not so thick as some others: it appears remarkably well adapted for the purpose of an altar. It is entirely surrounded by a number of upright stones, which were also covered by the carn.
I assume this is the right place.. I liked the manner of its discovery. The trouble is, elsewhere in the book is the passage it refers to (the book has instructional conversations in Irish and then the translation in English):
G. Was there not a cromleac found, under a carn, near that place [ie the place refered to in the note above]?
S. There was, indeed, about two miles from it, (about seven years before,) an exceeding large, broad, level, smooth stone, as polished as the pebbles on the sea coast: I am persuaded there is no other cromleac in Ireland so neat as it is; as the gentleman asserted, who came to view it.
There was an enclosure of long equal stones, standing strait up round the great cromleach, when it was found: under a great carn of small stones.
G. Were these long stones lifted?
S. They were all carried away to a building near the place.
G. Surely the cave was not broken.
S. It was broken and destroyed; neither flag nor stone was left, of any value, that was not carried away in the same manner.
G. I am surprised that the cave was broken.
S. why, even the round tower at Downpatrick was thrown down; and I think, Sir, that it is not lucky to touch such things.
Perhaps the cromleac would say, 'reports of my death are greatly exaggerated'. Maybe for purposes of learning vocabulary or dramatic effect?
From 'An introduction to the Irish language' by the Rev. W Neilson (1808) - it can be read on Google Books.