Carleton Jones book on the burren believes that this is a medieval inaugeration site for the O'Connors and O'Loughlins. He says that "Carn Mhic Tail is named after Tailm the ancestor of both Conchobhar and Locklainn, and it was considered the burial place of Tail". He does go on to say that many medieval inaugeration sites reused ancient burial mounds so as to legitimise their rule.
In this case I would think it would have to be a burial mound of some kind. It is a huge cairn, could be 10-12m high. No idea what diameter.
It is easy enough to get to, it is close to the little village of Kilshanny, dont try to cross the Derreen river to get to it, drive around to the opposite side by the road and it is only about two fields walk from the road in.