Fourth Declension

The fourth declension is one of the more straightforward noun classes in Irish. It primarily includes masculine nouns that end in a vowel, as well as all nouns with the diminutive suffix -ín. Its most defining feature is that the genitive singular form is identical to the nominative singular.

Characteristics of the Fourth Declension

This declension includes several distinct groups of nouns:

• Masculine nouns ending in a vowel: This is the largest group.
• Feminine nouns ending in a vowel: A smaller group of feminine nouns also belongs here.
• Nouns with the diminutive ending -ín: All nouns with this ending, regardless of their original declension, follow fourth declension rules.
• A few nouns ending in a consonant: This group includes common loanwords like bus, club, and tobac, as well as some native Irish words like ainm (name) and lucht (people).

Genitive Case

The key feature of the fourth declension is its simplicity in the singular.

• The genitive singular has the same form as the nominative singular.
• The genitive plural has the same form as the nominative plural.

This means that once you know the nominative singular and plural forms, you know all the case forms for this declension.

Plural Forms

Fourth declension nouns almost always have “strong” plurals, meaning the form does not change for the genitive plural. The plural is formed by adding a suffix, which depends on the noun’s ending.

Nouns ending in -ín

Nouns with the diminutive ending -ín add -í to form the plural.

• cailín → cailíní (girls)
• báidín → báidíní (little boats)

Nouns ending in -a or -e

• Nouns ending in -a change this to -aí.
  • dalta → daltaí (pupils)
  • slabhra → slabhraí (chains)
• Nouns ending in -e change this to -í.
  • file → filí (poets)
  • oráiste → oráistí (oranges)

Nouns ending in -le or -ne

Nouns ending in -le or -ne replace the final -e with -te.

• baile → bailte (towns)
• sloinne → sloinnte (surnames)

Nouns ending in a long vowel

• Nouns ending in -í, -aí, -aoi, or -é usually add the ending -the.
  • croí → croíthe (hearts)
  • garraí → garraíthe (gardens)
  • saoi → saoithe (wise people)

Other Plural Endings

A few common nouns have other plural endings:

• Ending -nna:
  • cú → cúnna (hounds)
  • fia → fianna (deer)
  • fleá → fleánna (feasts)
• Ending -(n)acha:
  • ainm → ainmneacha (names)

Irregular and Weak Plurals

A very small number of nouns in this declension have irregular or weak plurals. A weak plural changes its form in the genitive plural.

• bó (cow) → ba (cows) (Gen. Pl: na mbó)
• grásta (grace) → grásta (graces) (Gen. Pl: na ngrást)

Summary of Endings

This table summarises the most common patterns for the fourth declension.

Examples