I've always been led to believe that although stone circles may have been important to Druids they has no hand in their construction and basically inherited them. I find it interesting that in England there is a strong neo-Druidism whereas here in Ireland it is not so strong. I've yet to meet a neo-Druid at a stone circle in Ireland but always run into them in England. I think it's the strong Catholicism that still run throughout Ireland that has prevented this from happening.