Hi Dave 3×3.
The dating of ogham stones is an interesting topic, and not without some difficulties, either. A good number of the stones were prehistoric standing stones that had the ogham letters carved on to them at a much later date, obviously. The standing stones, or menhirs, are usually from the late Neolithic to early Bronze Age, thus pre-dating the use of ogham by millennia in some cases. As for the dating of the megaliths, dolmens and menhirs themselves, this is a subject for archaeologists and relies on factors such as archaeological context and the dating of material finds contemporary with the stones in question.
Regarding the dating of the ogham inscriptions themselves, well, linguistics can help out with this. Sound changes and language forms, principally in Archaic and Old Irish and then in Brythonic, Latin and Norse can all give an indication of the timeframe in which to place them. In addition to this, the subject matter, i.e. the names and so on, may also offer some indication. It’s important to note that ogham is first found in Ireland and seems to have spread out through parts of the British Isles, mostly the western areas, which came under Irish influence or which were colonised or invaded by Irish groups; ogham is not found throughout the Celtic world and nothing survives to suggest it dates from anything much earlier than the 4th century CE – the druids whom the Romans encountered would in all likelihood not known about ogham at all.