there is a stone in the british museum from the brecon beacons.it's called the "llywel" stone. it's got ogham down the side and on one face is a collection of carved pictures.
it's about 6 feet tall and thin, something like a thick door. it's flat on the top and bottom.
it looks to me like a pillar,just like is found in places like bryn celli ddu.the pictures are similar in style to the images from newgrange,although they have been interpreted as coming from the dark ages,
i think it is from a passage mound but it's origins are unknown,so its just my opinion.there is another ogham stone with latin in the church at defynnog,and this is the same area the llywel stone came from.