On my journey with the Dragon current through Pembrokeshire I was taken to this church, St. Rhian. I guess you mean the nicely inscribed stone in the church wall - which I suspect is really a map of the Llan's layout but placed in the wall up-side-down. (Photo added)
The road the church stands beside, from Carreg Samson all the way to St. Non's, is built on the Dragon - except for a very few places (this church building, for instance). There is also a marker stone on the road that indicates the Dragon's central flow from the road to the church through the southern part of the churchyard. After Y Ddraig's (the Dragon's) journey through the churchyard She flows through a farm building and the farmyard to exit through a stone that is serving as a gatepost. I will add photos of these too.
Something is not right. This message is just to keep things from messing up down the road