This phase of the language is known only from fragments, mostly personal names, inscribed on stone in the Ogham alphabet in Ireland and western Great Britain between the 4th and the 6th century AD,[6] before the advent of Old Irish.

[12] The scholastic use of Ogham continued until the early 19th century, the last inscription being found on the tomb of Mary Dempsey at Ahenny in County Tipperary, which is written in both Irish and English.

[13] Transcribed Ogham inscriptions, which lack a letter for /p/, show Primitive Irish to be similar in morphology and inflections to Gaulish, Latin, Classical Greek and Sanskrit.

Inscriptions demonstrating additional information are rare, such as QRIMTIR RON[A]NN MAQ COMOGANN, "[the stone] of the priest Ronán son of Comgán".

[17] The brevity of most orthodox ogham inscriptions makes it difficult to analyse the archaic Irish language in depth, but it is possible to understand the basis of its phonology and the rudiments of its nominal morphology.

[19] Furthermore, according to Damian McManus, Proto-Celtic nasal, dental, and velar stems also correspond to the Primitive Irish -AS genitive, attested in names such as GLASICONAS,[20] CATTUBUTTAS,[21] and LUGUDECCAS.

[16][23] There is a certain amount of obscurity in the vowel inventory of Primitive Irish: while the letters Ailm, Onn and Úr are recognized by modern scholars as representing /a(:)/, /o(:)/ and /u(:)/ respectively, there is some difficulty in reconstructing the values of Edad and Idad.

[27] The consonant inventory of Primitive Irish is reconstructed by Celticist Damian McManus as follows:[28][26] The letters Cért, Gétal and Straif, transliterated as Q, NG (or GG) and Z, respectively, were known by the ancient scholastic Oghamists as foilceasta (questions) due to the obsolescence of their original pronunciations: the first two, /kʷ/ and /ɡʷ/, had merged with plain velars in Old Irish, and the third, probably /st/, merged with /s/.

John T. Koch, an American Celticist, theorized that these changes coincide with the conversion of the island to Christianity and the introduction of Latin as a literary language.

This would give the impression of rapid linguistic development, while actually representing a shift in literature to a vernacular register which had previously been obscured by the conservative influence of the druidic language.

As an example, a 5th-century king of Leinster, whose name is recorded in Old Irish king-lists and annals as Mac Caírthinn Uí Enechglaiss, is memorialised on an Ogham stone near where he died.

[47] Similarly, the Corcu Duibne, a people of County Kerry known from Old Irish sources, are memorialised on a number of stones in their territory as DOVINIAS.

Gradually, the grammaticalization of consonant mutations introduced a new characteristic that Irish would eventually share with all other modern Celtic languages.