The Irish first declension is made up of masculine nouns. The nominative singular ends in a broad (velarized) consonant, which is made slender (palatalized) in the genitive singular. The vocative singular is identical to the genitive singular, and the dative singular is identical to the nominative singular.
The plural can be formed in any of a variety of ways, which can be grouped into two basic types: the weak plural forms and the strong plural forms.
A weak plural in Irish is a plural formation in which the genitive plural is distinct from the nominative plural. In the first conjugation, the weak genitive plural is always identical to the nominative singular. The weak nominative plural can be formed either by making the final consonant slender (for example, bád (“boat”) has nominative plural báid) or by adding -a to the broad consonant (for example, úll (“apple”) has nominative plural úlla). In both cases, the vocative plural is formed by adding -a to the broad consonant (e.g. báda, úlla), and the dative plural is identical to the nominative plural (however, in archaic language and some dialects the dative plural is formed by adding -aibh to the broad consonant, e.g. bádaibh, úllaibh).
| bád (“boat”) | Singular | Plural |
|---|---|---|
| Nominative | bád | báid |
| Vocative | a bháid | a bháda |
| Genitive | báid | bád |
| Dative | bád | báid (archaic) bádaibh |
When ch in the genitive singular and nominative plural of a polysyllabic word is made slender, it also becomes gh.
| marcach (“a horseman”) | Singular | Plural |
|---|---|---|
| Nominative | marcach | marcaigh |
| Vocative | a mharcaigh | a mharcacha |
| Genitive | marcaigh | marcach |
| Dative | marcach | marcaigh (archaic) marcachaibh |
Some nouns undergo a vowel change before the slender consonant of the genitive singular/nominative plural:
| úll (“apple”) | Singular | Plural |
|---|---|---|
| Nominative | úll | úlla |
| Vocative | a úill | a úlla |
| Genitive | úill | úll |
| Dative | úll | úlla (archaic) úllaibh |
Other examples:
A strong plural in Irish is a plural formation in which the all cases of the plural are identical (except for the archaic and dialectal dative plural in -(a)ibh). Many words of the first declension form a strong plural with one of the endings -(a)í, -ta, -tha, -anna.
| rós (“rose”) | Singular | Plural |
|---|---|---|
| Nominative | rós | rósanna |
| Vocative | a róis | a rósanna |
| Genitive | róis | rósanna |
| Dative | rós | rósanna (archaic) rósannaibh |
Other examples:
Other strong plural formations are found in: