Nominative

The nominative is used in the following functions:

1. Sentence subject

   Tá an cat ag ól. "The cat is drinking."

2. Sentence object

   Bhris Seán an fhuinneog. "Seán broke the window."

3. Predicate of the copula

   Is amadán é. "He is an idiot."

4. Object of the prepositions gan "without", go dtí "(up) to" and mar "like, as".

   gan an t-airgead "without the money"

   go dtí an t-am "(up) to the time"

   mar an chearc "like the hen"

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.

• Cá bhfuil tú, a mhic? "Where are you, son?"
• A Sheáin, tar anseo! "Seán, come here!"

Genitive

The genitive indicates possession and material of composition:

• hata an fhir "the man's hat"
• clann na mná "the woman's children"
• coinnleoirí an easpaig "the bishop's candelabras"
• fáinne óir "a ring of gold, a golden ring"
• bróga leathair "shoes of leather, leather shoes"

The object of a verbal noun also requires the genitive:

• ag caitheamh airgid "(the act of) spending money"

The object of a compound preposition is in the genitive. Formally, these prepositions are actually prepositional phrases.

• ar chúl an dorais "behind the door" (lit. "on the back of the door")
• ar feadh míosa "one month long" (lit. "for the duration of one month")
• ar son na hÉireann "for Ireland's sake"

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")).

• ag an athair "at the father"
• as an teach "out of the house"
• ar an arán "on the bread"
• in oráiste "in an orange"
• go hifreann "to hell"
• leis an airgead "with the money"
• ó Éirinn "from Ireland"

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 /x/ in the gen. sing. and nom. pl. of a polysyllabic word is made slender, it also becomes voiced, thus:

• /x/ > /ç/ > /j/. The resulting /əj/ is written -⟨(a)igh⟩ and is pronounced /iː/, /ə/, or /əɟ/, depending on dialect.

Some nouns undergo a vowel change before the slender consonant of the genitive singular/nominative plural:

• ball, baill - an (internal) organ, component part
• bonn, boinn - a sole, coin
• ceann, cinn - a head
• fear, fir - a man
• iasc, éisc - a fish
• mac, mic /mˠak, mʲɪc/ - a son (note: the first consonant is made slender in the gen.sg./nom.pl. as well)
• poll, poill - a hole

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:

• aonach, gen. sg. aonaigh, pl. aontaí - a fair
• bealach, gen. sg. bealaigh, pl. bealaí - a way
• carr /kaːɾˠ/, gen. sg. cairr /kaːɾˠ/, pl. carranna /kaɾˠən̪ˠə/ - a car
• glór, gen. sg. glóir, pl. glórtha - a voice
• leanbh, gen. sg. linbh, pl. leanaí - a child
• néal, gen. sg. néil, pl. néalta - a cloud
• rós, gen. sg. róis, pl. rósanna - a rose
• samhradh, gen. sg. samhraidh, pl. samhraí - a summer
• scéal, gen. sg. scéil, pl. scéalta - a story
• toradh, gen. sg. toraidh, pl. torthaí - fruit

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:

• ceart, gen. sg. cirt, nom. pl. cearta, gen. pl. ceart - a right
• cleas, gen. sg. clis, nom. pl. cleasa, gen. pl. cleas - a trick
• úll, gen. sg. úill, nom. pl. úlla, gen. pl. úll - an apple

Other strong plural formations are found in:

• bóthar, bóthair; bóithre - road
• breitheamh, breithimh; breithiúna - judge
• briathar, briathair; briathra - verb
• cloigeann, cloiginn; cloigne - skull
• doras, dorais; doirse - door
• ollamh, ollaimh; ollúna - professor
• solas, solais; soilse - light

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 /x/ in the gen. sing. is made slender, it is also voiced, so /x/ > /ç/ > /j/. /əjə/ becomes /iː/, 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:

• áit, áite, áiteanna "place"
• coill, coille, coillte "forest"
• iníon, iníne, iníonacha "daughter"
• obair, oibre, oibreacha "work"
• spéir, spéire, spéartha "sky"
• tír, tíre, tíortha "country"
• tonn, toinne, tonnta "wave"
• ubh, uibhe, uibheacha "egg"

Other strong plural formations are found in:

• fiacail, fiacaile; fiacla - tooth
• gualainn, gualainne; guaillí - shoulder
• scian, scine; sceana - knife (irregular genitive singular)
• sliabh, sléibhe; sléibhte (m.) - mountain (irregular genitive singular and masculine gender)

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 ⟨t⟩ in the gen. sg.; those in -irt have -⟨th⟩- instead of -⟨t⟩- in the gen. sg.

• bagairt, bagartha, bagairtí (f.) "threat"
• canúint, canúna, canúintí (f.) "dialect"

Many words in this declension form the plural with one of the endings -anna or -acha:

• am, ama, amanna (m.) "time"
• anam, anama, anamacha (m.) "soul"
• droim, droma, dromanna (m.) "back"
• loch, locha, lochanna (m.) "lake"
• troid, troda, troideanna (f.) "fight, struggle"

Some words in Munster Irish also have a separate dative form:

• nom. drom, dat. droim, gen. droma, pl. dromanna (m.) "back"

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:

• ainmhí; ainmhithe (m.) "animal"
• aturnae; aturnaetha (m.) "attorney"
• baile; bailte (m.) "village"
• bus; busanna (m.) "bus"
• cliamhain; cliamhaineacha (m.) "son-in-law"
• cneá; cneácha (f.) "wound, sore"
• cnó; cnónna (m.) "nut"
• cró; cróite (m.) "outhouse; eye of a needle"
• dlí; dlíthe (m.) "law"
• dosaen; dosaenacha (m.) "dozen"
• ga; gathanna (m.) "ray, radius"
• gé; géanna (f.) "goose"
• léine; léine, léinte (f.) "shirt"
• rá; ráite (m.) "saying"
• rí; ríthe (m.) "king"
• sloinne; sloinnte (m.) "last name"
• teanga; teangacha (f.) "language, tongue"
• tine; tinte (f.) "fire"

Other strong plural formations are found in:

• ainm; ainmneacha (m.) "name"
• airí; airíona (m.) "characteristic, symptom"
• aithne; aitheanta (f.) "commandment"
• bruach; bruacha (m.) "bank (of river etc.)"
• cine; ciníocha (m.) "race, tribe"
• duine; daoine (m.) "person, human being"
• gabha; gaibhne (m.) "blacksmith"
• gnó; gnóthaí (m.) "business"
• oíche; oícheanta (f.) "night"

One noun in this class has a weak plural:

• bó, bó; ba, bó (f.) - cow

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". In Éire, Éireann "Ireland" the dative Éirinn is still used 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.

• abhainn, abhann, aibhneacha "river"
• athair, athar, aithreacha (m.) "father"
• deartháir, dearthár, deartháireacha (m.) "brother"
• máthair, máthar, máithreacha "mother"

Other strong plural formations are found in:

• bráthair, bráthar; bráithre (m.) "brother (monk), friar"
• cara, carad; cairde (m.) "friend"
• namhaid, namhad; naimhde (m.) "enemy"
• Nollaig, Nollag; Nollaigí "Christmas"

Some nouns have weak plurals; here the genitive singular and genitive plural have the same form:

• caora, caorach; caoirigh, caorach - sheep
• lacha, lachan; lachain, lachan - duck

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.

• briseadh, briste; bristí "breaking"
• moladh, molta; moltaí "praising; recommendation"

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.

• scrúdú, scrúdaithe; scrúduithe "examining, examination"
• síniú, sínithe; sínithe "stretching"

Irregular nouns

The following nouns are declined irregularly:

• bean, mná; mná, ban (f.) "woman"
• deirfiúr, deirféar; deirfiúracha (f.) "sister"
• deoch, dí; deochanna (f.) "drink"
• Dia, Dé; déithe (m.) "God"
• lá, lae; laethanta (m.) "day"
• leaba, leapa; leapacha (f.) "bed"
• mí, míosa; míonna (f.) "month"
• muir, mara; mara (f.) "sea"
• olann, olla (f.) "wool"
• talamh, talaimh (m.) or talún (f.); tailte "land"
• teach, tí; tithe (m.) "house"

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.

• Ba thréine Cáit ná Cathal. "Cáit was stronger than Cathal."
• Is airde Seán ná mise. "Seán is bigger than me."
• B'óige an madra ná an cat. "The dog was younger than the cat."
• Is fearr Gaeilge bhriste ná Béarla cliste. "Broken Irish is better than clever English."

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.

• Tá an ghrian níos gile ná an ghealach. "The sun is brighter than the moon."
• Beidh Peadar níos saibhre ná a athair. "Peadar will be richer than his father."
• D'éirigh Peadar ní ba shaibhre ná a athair. "Peadar became richer than his father."
• Bhí Seán ní b’airde ná mise. "Seán was bigger than me."

A superlative is expressed as a relative clause: noun + is/ba/ab + comparative form.

• an cailín is tréine "the strongest girl" (lit. "the girl who is the strongest")
• an cailín ba thréine "the strongest girl" (lit. "the girl who was/would be the strongest")
• an buachaill is óige "the youngest boy" (lit. "the boy who is the youngest")
• an buachaill ab óige "the youngest boy" (lit. "the boy who was/would be the youngest")