Overview

An overview of the five main noun declensions in Irish, classified by how they form the genitive singular case.

Nouns Declension Grammar Cases

In Irish, nouns change their form depending on their grammatical function in a sentence. This system of changing endings is called declension. Nouns are grouped into five main declensions based on how they form their genitive singular, which is the case used to show possession, similar to “of the…” in English.

The primary feature that defines a noun’s declension is the form of its genitive singular. The dative singular, though less common in modern speech, also often follows a predictable pattern for each declension. Plural forms, however, are much more varied and are not used to classify nouns.

A small number of common nouns have highly irregular forms and are grouped separately. Additionally, verbal nouns ending in -adh or -ú follow a regular pattern where their genitive form is the same as the verbal adjective.

Overview of Declensions

The following sections summarise the main patterns for each declension.

Main Declensions

The first four declensions have the most regular genitive singular patterns.

Declension	Typical Gender	Genitive Singular Pattern	Dative Singular Pattern	Example (Nom. - Gen. - Dat.)
1st	Masculine	The final consonant becomes slender (palatalisation).	Same as the nominative.	`fear` - `fir` - `fear`
2nd	Feminine	The final consonant becomes slender and adds `-e`.	The final consonant becomes slender.	`long` - `loinge` - `loing`
3rd	m. or f.	Adds `-(e)a`. The final consonant may become broad.	Same as the nominative.	`leann` - `leanna` - `leann`
4th	Masculine	No change from the nominative.	Same as the nominative.	`cailín` - `cailín` - `cailín`

Fifth Declension Groups

The fifth declension contains several smaller patterns, many of them historical stem types.

Group	Typical Gender	Genitive Singular Pattern	Dative Singular Pattern	Example (Nom. - Gen. - Dat.)
Group 1	m. or f.	The final consonant becomes broad (depalatalisation).	Same as the nominative.	`athair` - `athar` - `athair`
Group 2	Feminine	Adds `-(e)ach`.	Adds `-(a)igh`.	`beoir` - `beorach` - `beoraigh`
Group 3	Feminine	Adds a broad `-n` or `-nn`.	Adds a slender `-n` or `-nn`.	`Éire` - `Éireann` - `Éirinn`
Group 4	Masculine	Adds a broad `-d`.	Adds a slender `-d`.	`cara` - `carad` - `caraid`

Special Patterns

Some nouns do not fit neatly into the five main declension patterns.

Pattern	Typical Gender	Genitive Singular Pattern	Dative Singular Pattern	Example (Nom. - Gen. - Dat.)
Irregular nouns	m. or f.	Follows no standard pattern.	Follows no standard pattern.	`bean` - `mná` - `mnaoi`
Verbal nouns ending in `-adh` or `-ú`	Masculine	Uses the form of the verbal adjective.	Same as the nominative.	`bualadh` - `buailte` - `bualadh`

Historical note on Latin parallels

The classification of Irish declensions shows some parallels with Latin.

1st Declension: Compare Latin vir (gen. viri) with Irish fear (gen. fir). The Latin genitive ending -i of the o-declension corresponds to the palatalisation of the final consonant in Irish.
2nd Declension: Compare Latin longa (gen. longae) with Irish long (gen. loinge). The Latin genitive ending -ae of the a-declension corresponds to the Irish ending -e along with palatalisation.