On my pilgrimages I have seen several examples of stones with ogham inscriptions on them. Ogham was an early form of writing in use in south -east Ireland from the fourth to the sixth centuries, being used to write down early Irish. Ogham looks rather like a modern barcode and was inscribed using the vertical edge of an upright stone as a base line. It appears to have been influenced by Roman numerals and probably reflects the influence of late Roman Gaul on Ireland although Ireland was never, of course, part of the Roman Empire.
Waves of settlement from Ireland from the late fourth century have resulted in ogham stones being found in western Britain from Scotland and the Isle of Man to Cornwall and Devon.
However Wales, with its close proximity to south east Ireland, is particularly rich in ogham stones of which there are 35 undisputed examples. These are mainly in two clusters, that is in south west Wales and the Brecon area of mid-Wales. In Wales, ogham inscriptions are often combined with the same words in Latin, typically reading X son of Y. The ogham and Latin inscriptions are believed to have been made concurrently.
The stones are believed to be grave or boundary markers and to mark a society which was becoming Christianised. Many are in churches or churchyards, which are themselves ancient Christian sites, either for safekeeping or because they have at some point been built into the church building. Some are in museums although a few are still in what is believed to be their original position.
I also saw ogham stones on my pilgrimage to the Isle of Man where ogham seems to have remained in use until the Viking period and was sometimes combined with Norse runes.