In Gaulish and the Brythonic languages, a new /p/ sound has arisen as a reflex of the Proto-Indo-European /k /phoneme. In so far this new /p/ fills the space in the phoneme as a chain shift in Old Irish. Precise fortis sonorants in Ogham articulation is unknown, which were probably longer, tenser, than their lenis counterparts. Transcribed Ogham inscriptions show Primitive Irish to be archaic in character, lacking a letter for the /p/ phoneme, and in morphology and inflections similar to Gaulish, Latin, Classical Greek or Sanskrit.

European voiced aspirated stops {b, d, g} merged. The consonant inventory of the Tree alphabet Old Irish is /N/, /N?/, /L/, /L?/, /R/, /R?/ represent fortis sonorants whose precise articulation is unknown.

 

Personal pronouns as direct objects are infixed between the preverb and the verbal stem, along with various other particles that modify the verb's meaning (including the negative) or indicate certain special sentence structures. Most verbs have, in addition to the tenses, voices, and moods named above, two sets of forms: a conjunct form, and an absolute form.

As with most medieval languages, the orthography of Old Irish is not fixed, so the following statements are to be taken as generalizations only; individual manuscripts may very greatly.