Marked on Explorer maps, but not on Landranger maps, RCAHMS says that this is "A prostrate stone, over 6' long" and that it is "covered with cup-markings". It was in mint condition in 1882, but was more recently said to bear only faint cups. No indication of how many.
Unfortunately, This lad didn't get to check it out. I'd like to visit again, but it's unlikely, as this place really is the back of beyond. That's a big part of why it's such a nice spot I guess.