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".

A number of different numbering schemes are used. The most widespread is CIIC, after R. A. S. Macalister (Corpus Inscriptionum Insularum Celticarum, Latin for "corpus of Insular Celtic inscriptions"). This covers the inscriptions known by the 1940s. Another numbering scheme is that of the Celtic Inscribed Stones Project, CISP, based on the location of the stones; for example CIIC 1 = CISP INCHA/1. Macalister's (1945) numbers run from 1 to 507, including also Latin and Runic inscriptions, with three additional added in 1949. Ziegler lists 344 Gaelic ogham inscriptions known to Macalister (Ireland and Isle of Man), and seven additional inscriptions discovered later.

The inscriptions may be divided into "orthodox" and "scholastic" specimens. "Orthodox" inscriptions date to the Primitive Irish period, and record a name of an individual, either as a cenotaph or tombstone, or documenting land ownership. "Scholastic" inscriptions date from the medieval Old Irish period up to modern times.

The vast bulk of the surviving ogham inscriptions stretch in arc from County Kerry (especially Corcu Duibne) in the south of Ireland across to Dyfed in south Wales. The remainder are mostly in south-eastern Ireland, eastern and northern Scotland, the Isle of Man, and England around the Devon/Cornwall border. The vast majority of the inscriptions consists of personal names, probably of the person commemorated by the monument.