Roughly 400 known ogham inscriptions are on stone monuments scattered around the Irish Sea, the bulk of them dating to the fifth and sixth centuries. Their language is predominantly Primitive Irish, but a few examples record fragments of the Pictish language. Ogham itself is an Early Medieval form of alphabet or cipher, sometimes known as the "Celtic Tree Alphabet".

Ogham is an Early Medieval alphabet used primarily to write the early Irish language, and later the Old Irish language. There are roughly 400 surviving orthodox inscriptions on stone monuments throughout Ireland and western Britain, the bulk of which are in southern Munster. The largest number outside Ireland are in Pembrokeshire, Wales.