You know, I saw that stone and thought 'ohh, a bullaun!' but then went 'naahh'. Its just outside the fence at the north east corner of the enclosed area. Its not a very impressive bullaun, I only noticed it because there was a rectangular stone sitting in it, it was full of water and muck. I'd say its about 12" x 10" in size and about 3-4" deep but the sides are fairly gently sloping. The stone its in is almost a perfect semi-circle at the top and the bottom is flattened I think (its sunk into the mud here).

Those oddly carved stones could well be the base of timber or metal posts, maybe the carved slab was covered by some kind of roofing at one point. The link I gave did say there were grooves in the top of the slab itself that probably had something slot into them as well. I'll stick some pics into the next weblog.