Notable characteristics of Old Irish compared with other old Indo-European languages, are:
- Initial mutations, including lenition, nasalisation and aspiration/gemination.
- A complex system of verbal allomorphy.
- A system of conjugated prepositions that is unusual in Indo-European languages (although they are found in many Semitic languages such as Arabic): dím "from me", dít "from you", de "from him", di "from her", diib "from them" (basic preposition di "from"). There is a great deal of allomorphy here, as well.
- Infixed object prepositions, which are inserted between the verb stem and its prefix(es). If a verb lacks any prefixes, a dummy prefix is normally added.
- Special verbal conjugations are used to signal the beginning of a relative clause
Old Irish also preserves most aspects of the complicated Proto-Indo-European (PIE) system of morphology. Nouns and adjectives are declined in three genders (masculine, feminine, neuter); three numbers (singular, dual, plural); and five cases (nominative, vocative, accusative, dative and genitive). Most PIE noun stem classes are maintained (o-, yo-, ā-, yā-, i-, u-, r-, n-, s-, and consonant stems). Most of the complexities of PIE verbal conjugation are also maintained, and there are new complexities introduced by various sound changes (see below).