A noun is in the genitive when it directly follows another noun. This rule applies in instances both when the definite article is present and when it is not. Nouns in the genitive qualify the preceding noun in the nominative, giving additional information about its attributes, such as time, place, quantity, possession and so forth.
fa chríochuibh an domhuin mhóir (PCT) [masculine genitive singular]
go Loch na nDeamhan (PCT) [masculine genitive plural]
i gCuigeadh chlann Rudhraidhe (Cú Chulain an chroidhe mhir, quatrain 28) [example of genitive plural where eclipsis would be expected, but where sléagar allows the poet to use lenition]
Fir Éireann d’aonrún cogaidh (Cú Chullain an chroidhe mhir, quatrain 29)
In bardic poetry, the nominative can sometimes be used for the genitive of proper nouns and for the genitive of some common nouns. [Iomarbhágh na bhFileadh 1, p. xx; references section 5, quatrains 35, 57, 177]
is osnadha fhir gona (Iomarbhágh na bhFileadh, section 2, quatrain 35) [nominative plural fir used for genitive plural fear]
ar chosg draoidheacht druadh Cormac (Iomarbhágh na bhFileadh, section 5, quatrain 57) [nominative singular draoidheacht used in place of genitive singular draoidheachta]
When a noun in the genitive qualifies a preceding noun by describing its attributes, it is often referred to as an attributive genitive. Attributive genitives function like adjectives by describing various attributes of the noun they follow.
A noun is also in the genitive when it follows a verbal noun. In Modern Irish, the genitive noun following a verbal noun is often described as the “object of the verbal noun,” but in the study of Early Modern Irish, both the verbal noun and the noun(s) following it are analyzed in terms of the genitive.
Examples of noun following a verbal noun:
ag aghmhilleadh bhfear nÉirionn (PCT) [both nouns are in the genitive relative to the verbal noun aghmhilleadh]
do lánmhilleadh toraidh na talmhan (PCT) [both nouns are in the genitive relative to the verbal noun lánmhilleadh]
ag bronnadh séad (“Brian Ó Ruairc,” quatrain 3) [example of a genitive plural that has the same form as the nominative singular, which in Modern Irish is referred to as lagiolra.]
Conchubhar, rí Uladh, do chaitheamh fleidhe (Keating, “Marbhadh,” paragraph 2)
The partitive genitive occurs when a genitive noun follows a noun or adverb describing quantity.
ní cuid amhrais annséin (Tadhg Dall Ó hUiginn, The Bardic Poems of Tadhg Dall Ó hUiginn, poem 7, quatrain 4)
Instances when the genitive might be expected, but is not used:
The genitive form (spelling) is not used after cardinal or ordinal numbers, except the number one.
Ar gclos an dara mairge (Keating, FFÉ, Section 42) [an example where Keating contravenes this rule and uses mairge the genitive of mairg]
The genitive form (spelling) is also not used for feminine proper names when they are followed by an adjective, nor when used with a surname.
Though certain prepositions (so-called “compound prepositions”) require the genitive for the following noun, the following noun does not take the genitive form (spelling) when it is part of an infinitive construction.