When they get that big they start to fall into the Bullaun stone catagory. That's another 'out of Ireland' one to add to my list. Irish Archaeos tend to see them as an Irish thing - actually, that's not quite true ... most Irish archaeos don't give them a second thought! Most reckon they are Medieval at best. A few will allow them to be Iron Age. I am the first to propose a Neolithic origin for the Irish ones and the first to really start looking outside of Ireland.