The most ancient inscriptions in Ogham date from the 3rd century AD, but most of them were written in the 5-9 centuries. Interesting that the Irish remembered Ogham even in the previous century, after a thousand years of using the Roman alphabet. The total of inscriptions makes about 500, of them about 50 ones are written both in Ogham and in Roman and found in Wales and Cornwall. These are most important for the study of the Ogham alphabet and its varieties, as well as the Irish language, which went through its crucial moment just in the 4-5th centuries AD.
Most inscriptions are horribly boring with their form of "X son of Y" on corners of large stone slabs. But still they keep rich material for Celtic language studies. Originally Ogham consisted of 20 symbols, later 5 more were added. It could be either vertical or horizontal, but always looked like dots and short lines placed on both sides of an axis.
After the 10th century the Irish take up the Roman alphabet, and Ogham is used only in private life in distant regions of Ireland. Another situation with around 30 inscriptions from Scotland, which are written in an unknown language, sometimes called Pictish. Though we can read them, no translation is found, as Pictish was likely non-Indo-European. Pictish documents disappear in the 9th century, with the assimilation of the Picts by Scots.
Languages which used the script:
Celtic (Old Irish, Brittish or Old Welsh), non-Indo-European (Pictish).