I found a picture of this stone in a newsletter of the South East Galway Archaeological and Historical Society. It looks nice. And there's a story to go with it:
There is a remarkable upright stone in the townland of Knockroe in the parish of Ballinakill.
It stands about eight feet over the ground and is about four feet wide by one and a half in thickness.
The local belief is that a famous giant threw it from Knockash hill a distance of about three miles. They say only for his foot slipped when he was throwing it he would lodge it on the Ben Hill about two mile further on to the south.
It is supposed that when he slipped he knocked a piece out of the mountain and the gap is there ever since.