The tipp inventory gives it dims as (diam 0.26m: D 0.11m) which sounds about right. The stone itself is in the North Eastern quadrant of the ring-fort.
Its an unusual stone in that there is no evidence that I know of any early religious sites within the vicinity. The stone itself is tricky to find. Its off a back-road near Borrisoleigh, I stopped at a wall with a no dumping sign that is adjacent to a ring-fort up on the summit of a hill (on opposite side of road to the wall). (If you find this spot you will know it!)
Then the stone is about 3 fields in directly to the rear of a farm stead but just far enough away that you wont look too suspicious walking around.