Clonmacnoise in central Ireland.
Founded as a monastic settlement by St. Ciaran (Kieran) in 548 CE, this site was important during the fifth and sixth centuries CE, the golden age of the Irish saints. In 1198, Rory O'Connor, Ireland's last High King, was interred here.
The site was likely chosen because it lies at a conjunction of ley lines. These magnetic earth lines were important in ancient Druidic religion. Many ancient monuments and ruins including Newgrange and the Hill of Tara in Ireland, as well as Rosslyn Chapel in Scotland and Stonehenge and Avebury in England are located on ley lines. Today Jackie Queally is one travel guide who explains such sacred sites.
Like many Irish monastic settlements, Clonmacnoise was attacked and plundered on several occasions by Anglo-Normans and Vikings. Yet in spite of its violent past, the place is imbued with a mysterious sense of power and peace.
A good way to view this vast site is to begin by watching the short film that introduces its history.