The Irish fifth declension is made up primarily of feminine nouns; however, male familial nouns (e.g. athair), and the cardinals 20, 30 etc. (e.g. fiche, tríocha) are masculine. The nominative singular can end in: slender consonants ‑id, ‑in(n), ‑ir (seldom ‑ig, ‑il); a vowel.
The fifth declension is characterised by the genitive singular having a broad consonant (depalatised). The dative singular tends to be slender (palatised).
Fifth declension plurals are predominantly strong plural with a few exceptions.
The vocative forms, both singular and plural, are the same as the nominative.
There are relatively few nouns in the fifth declension.
The general rule for fifth declension genitive singular is that it end in a broad consonant. This can occur by depalatisation of a slender consonant root, usually with the addition of ‑ach; or addition of ‑d or ‑n(n) for a vowel root.
- depalatisation (broadening)
- depalatisation + -ach
- slender radical + -each (with syncopation if necessary)
- vowel + -d
- vowel + -n(n)
Fifth declension nouns form the dative singular by taking the genitive singular and slenderising (palatisation). However, since most have radicals with slender consonants, the form is difficult to spot.
The most accessible example is Éire:
Other examples in common use
For a few fifth declension nouns ending with ‑in(n), the nominative singular is in fact the original dative singular.
A weak plural is characterised by the genitive plural having the same form as the nominative singular. There are arguably no nouns classed as such in the fifth declension.
A strong plural is characterised by the genitive plural maintaining the same form as the nominative plural.
The regular way to form strong plurals in the fifth declension is to take the genitive singular form and append ‑a.
A few nouns with broad-consonant genitive ending, slenderise (with possible syncopation) and add ‑eacha
- athair m (“father”), athar, aithreacha
- deartháir m (“brother”), dearthár, deartháireacha
- máthair f (“mother”), máthar, máithreacha
- abha/abhainn f (“river”), abhann, aibhneacha
A few nouns have strong plurals by slenderising the genitive (if necessary) and adding (i)‑í
- díle f (“flood”) , díleann, dílí
- Nollaig f (“Christmas”) , Nollag, Nollaigí
- The cardinals:
A full bare-form example is shown below for cathair.
|Vocative||a chathair||a cathracha|
The fifth declension has some "weak"-ish plurals, where the genitive plural is the same as genitive singular.
- cara m (“friend”), gs. -d, ds. -id, npl. cairde, gpl. -d (also strong)
- caora f (“sheep”), gs. -ch, ds. -igh, npl. -igh, gpl. -ch
- comharsa f (“neighbour”), gs. -n, ds. -in, npl. -na, gpl. -n
- dearca f (“acorn”), gs. -n, ds. -in, npl. -in, gpl. -n
- eo m (“salmon”), gs. iach, ds. iaich, npl. iaich, gpl. iach (lit.)
- faocha f (“periwinkle”), gs. -n, ds. -in, npl. -in, gpl. -n
- lacha f (“duck”), gs. -n, ds. -in, npl. -in, gpl. -n
- pearsa f (“person”), gs. -n, ds. -in, npl. -na, gpl. -n
Other Irregular plurals:
- bráthair, gs. bráthar, npl. bráithre, gpl. bráithre
- ionga, iongan, ingne, ingne
- talamh, talún, tailte, tailte
As a general rule, nouns of the fifth declension are feminine.
The masculine nouns are, a) the male familials:
- aire m (“chief”)
- athair m (“father”)
- bráthair m (“brother”)
- cara m (“friend”)
- deartháir m (“brother”)
- namhaid m (“enemy”) 
And b) the cardinals 20, 30, 50, 60, 70, 80, 90
- fiche m (“twenty”)
- tríocha m (“thirty”)
- caoga m (“fifty”)
- seasca m (“sixty”)
- seachtó m (“seventy”)
- ochtó m (“eighty”)
- nócha m (“ninety”)
Multiple declension nouns
- For a list of nouns which have fifth and other declension forms, see the multiple declension table in the Irish nouns appendix.
- For fifth declension nouns ending in a vowel, see the list in the fourth declension appendix.
- An Caighdeán Oifigiúil, 2017
- A Grammar of the Irish Language, O'Donavan, 1845
- Irish declension on Wikipedia
- Irish declension on WikiBooks, with exercises
- Declension overview and guesser on Nualéargais
- Irish declension overview on Project Gutenberg
- Appendix:Irish nouns
Category:Irish appendices - Nouns 5