Nominative
The nominative is the default position for nouns. The nominative singular form is the same as the dictionary head-word form of the noun.
Some examples:
do bhí cogadh agus coinbhliocht idir Chonnachtaibh agus Ultaibh (Keating, “Marbhadh”)
Bíodh a fhiadhnaise sin ar mhórán d’uaislibh (Keating, TBB)
ní fís bhréige í (Síogaí Romhánach)
slat do b’orsa d’fhine Ír (Cú Chulainn an chroidhe mhir)
For more on how the nominative behaves, see the sections on initial mutation and inflection: