The declension of Irish nouns, the definite article, and the adjectives is discussed on this page (for pronouns, see Irish Grammar).

Nouns

Gender

Nouns in Irish are divided into two genders, masculine and feminine; the Old Irish neuter gender no longer exists. While gender should be learned when the specific noun is learned, there are some guidelines that can be followed: Generally, nouns in singular form ending with broad consonants are masculine, while those ending in a slender consonant are feminine. There are some exceptions, mostly dealing with specific endings and suffixes; for example, words ending in -óir/-eoir and -ín (with a slender and respectively) are categorically masculine, while words ending in -óg/-eog (with a broad ) are feminine. This leads to some unexpected gender assignments, such as gasóg "boy scout" being feminine, and cailín "girl" masculine (the diminutive -ín suffix is always masculine irrespective of the noun it applies to).

Case

Irish has four cases: common (usually called the nominative, but it covers the role of the accusative as well), vocative, genitive, and the dative or prepositional case.

Nominative

The nominative is used in the following functions:

Vocative

The vocative is used in direct address, and is always preceded by the particle a, which triggers lenition (the vocative particle is not pronounced before a vowel sound). The first declension is the only declension in which the vocative is distinct from the nominative.

Genitive

The genitive indicates possession and material of composition: The object of a verbal noun also requires the genitive: The object of a compound preposition is in the genitive. Formally, these prepositions are actually prepositional phrases.

Dative/Prepositional

The dative/prepositional is used as the object of most simple prepositions except gan and go dtí. In standard language, the dative is almost always identical to the nominative. Some dialects, however, have distinct standalone datives in the second and fifth declensions. In the standard language, only two words Éire ("Ireland") and fiche ("twenty") have distinct datives - Éirinn and fichid, respectively. They are also found in certain fixed phrases with nouns of the second declension, such as os cionn ("above", lit. "over head" – cionn is the old dative of ceann ("head")).

Declension

There are five recognized declensions in Irish. The makeup of the declensions depends on three factors: The following chart describes the characteristics of each declension class:

First

The first declension is made up of masculine nouns. The nominative singular ends in a broad consonant, which is made slender in the genitive singular. The most common formation of the plural has the opposite pattern: the nominative ends in a slender consonant, the genitive in a broad consonant (these plurals are known as weak plurals in comparison with strong plurals which maintain identical endings for all cases in the plural). The dative is identical to the nominative in both numbers, although an obsolete dative plural in -aibh is still sometimes encountered in old-fashioned literary style. When in the gen. sing. and nom. pl. of a polysyllabic word is made slender, it also becomes voiced, thus: Some nouns undergo a vowel change before the slender consonant of the genitive singular/nominative plural: Many words of this declension form the plural with one of the endings -(a)í, -ta, -tha, -anna. These are known as "strong plural" endings, which means the plural is identical in all cases in the standard language. Some examples: Some nouns have a weak plural (a plural where the genitive is different from the nominative, and is identical to the form of the nominative singular) in -a: Other strong plural formations are found in:

Second

The second declension is made up of mostly feminine nouns, and features a nominative singular form that can end in either a broad or a slender consonant. The genitive singular ends in a slender consonant followed by -e. The most common plural form has a broad consonant followed by -a in the nominative, and a broad consonant alone in the genitive. The vocative has the same endings as the nominative, as does the dative in standard language. In Connacht Irish and Waterford Irish it is often the case that all nouns of the second declension in the nom. sg. end with a slender consonant (e.g. bróig "a shoe"). In some Munster varieties as well as the old literary language, the dative singular is distinct and ends in a slender consonant alone (in effect the dative sg. is formed by dropping the -e from the genitive sg.), e.g. i mo bhróig "in my shoe" (historically, nominative forms like bróig are descended from the old dative). When in the gen. sing. is made slender, it is also voiced, so > >. becomes, and is written -(a)í. Many words in this declension form a strong plural with one of the endings -t(h)a,-te, -(e)acha or -eanna: Other strong plural formations are found in:

Third

The third declension is made up of masculine and feminine nouns. It is characterized by the genitive singular in -a. The majority of nouns in this class form the plural in -(a)í. The final consonant of the stem may be broad or slender: it retains its quality in the plural, but is always broad in the genitive singular. Feminine nouns in -áint or -úint lose their in the gen. sg.; those in -irt have -- instead of -- in the gen. sg. Many words in this declension form the plural with one of the endings -anna or -acha: Some words in Munster Irish also have a separate dative form:

Fourth

The fourth declension is made up of masculine and feminine nouns. It is characterized by a genitive singular that is identical in form to the nominative/vocative/dative singular. The singular may end in a vowel or a consonant (usually the diminutive suffix -ín). The most common plural ending is -(a)í. Many words of this declension form the plural with the following endings -tha/-t(h)e, -((e)a)nna or -((e)a)cha: Other strong plural formations are found in: One noun in this class has a weak plural:

Fifth

The fifth declension is made up mostly of feminine nouns and is characterized by a genitive singular that ends in a broad consonant that has been added to the nominative/vocative/dative singular. The most common plural is strong, formed by adding -a to the genitive singular. In some Munster Irish varieties as well as the old literary language, the dative singular is distinct and ends in a slender consonant (in effect the dative sg. is formed by palatalizing the genitive sg.), for example, do phearsain "to a person", ón gcathraigh "from the city". The word Éire ("Ireland") retains the distinct dative form Éirinn in the standard language. Some words form the genitive singular by changing the final consonant of the nominative singular to broad. The plural is then strong -eacha. Other strong plural formations are found in: Some nouns have weak plurals; here the genitive singular and genitive plural have the same form:

Verbal nouns

The most productive verbal nouns end with -(e)adh (1st conjugation) or -(i)ú (2nd conjugation). These originally belonged to the third declension, but synchronically are best regarded as separate declensions. The 1st conjugation verbal noun in -(e)adh has a genitive singular in -te/-ta and a plural in -t(a)í. The 2nd conjugation verbal noun in -(i)ú has a genitive singular in -(a)ithe and a plural in -(u)ithe. These endings are pronounced the same regardless of the spelling distinction.

Irregular nouns

The following nouns are declined irregularly:

Articles

The definite article has two forms in Irish: an and na. Their distribution depends on number, case, and gender, and they trigger mutation partly on the basis of the initial sound of the following word. Each entry of the table gives an example of one noun starting with a consonant and one with a vowel. Dative (i) is used with all prepositions in Ulster usage; in Munster and the standard language it is used only with den "from the", don "to the", and sa(n) "in the" but there are also Munster dialects in which only sa(n) triggers lenition and den and don eclipse, as with every other article-preposition compound. In Connacht sa(n) eclipses whereas den and don lenite. Dative (ii) is used outside Ulster with other prepositions. The article never mutates a following or in the singular, and is lenited to (pronounced ) rather than the usual. furthermore lenites in both dative (i) and (ii) in the singular with feminine nouns but does not lenite at all with masculine nouns. It does, however, eclipse and in Munster dialects and forms like "ag an ndoras" instead of the usual pattern "ag an doras", which is used in all other dialects, do occur. There is no indefinite article in Irish, so depending on context cat can mean "cat" or "a cat".

Adjectives

Almost all adjectives in Irish can be used either predicatively or attributively. A predicative adjective is one that forms a part of the predicate, like red in the sentence The car is red. An attributive adjective directly modifies a noun, as in the red car. A predicate adjective in Irish does not inflect: A predicate adjective expressing a value judgment is often preceded by the particle go. This particle attaches to a following vowel. In Ulster, go is not generally used in these cases. An attributive adjective mostly follows the noun and is inflected: There are four classes of declension of adjectives in Irish, which correspond to the first four declensions of nouns:

First declension

Second declension

Third declension

Fourth declension

This declension does not inflect, but it does mutate.

Irregular adjectives

Comparative

Irish adjectives have a comparative form equivalent to the comparative and superlative in English. The comparative does not undergo inflexion and is the same as the feminine singular genitive in regular and many irregular adjectives.

Regular formation

Irregular forms

Syntax of comparison

There are two constructions to express the comparative:

1. Copula + comparative form + subject + ná ("than") + predicate. The preterite of the copula causes lenition, while the present tense does not.
2. níos/ní ba/ní b’ + comparative + ná + predicate. Níos is used if the sentence is in the present or future tense. Ní ba/ní b’, which triggers lenition, is used if the sentence is in the past tense. Ní b’ is used before words starting with vowels and ní ba before those starting with consonants. A superlative is expressed as a relative clause: noun + is/ba/ab + comparative form.