Caibidil a hAon: The Noun (an tAinmfhocal)

what belongs to it ?

• mostly feminine nouns
• ending in slender consonants (with i as the last vowel: -il,-in,-inn,-ir) or
• ending in a vowel
• terms showing family relations (athair, máthair, bráthair)

forming the genitive:

The common denominator is that the genitives end in a broad consonant.

• depalatalisation: athair - athar (r-stems), Albain - Alban, abhainn - abhann (n-stems), Nollaig - Nollag
• depalatalisation + -ach: many nouns ending in -il,-in,-ir or a vowel:  beoir - beorach, cathair - cathrach
  partially without depalatalisation + -each (polysyllabic nouns with slender consonants in the 2nd syllable): litir - litreach,
  often syncopation (shortening) of the 2nd syllable: litir - litreach, uimhir - uimhreach (etym. ch-stems)
• nouns ending in a vowel + -n, -d: cara - carad, pearsa - pearsan, caoga - caogad  (etym. n- and nt-(d-)stems)

dative:

as a side note :
Here, the common denominator is that all dative forms end in a slender consonant:

• many of the words ending in -inn, -in are originally dative forms that actually have replaced the nominative form which ended in a vowel: Éire (colloquially more often Éirinn), Albain (in Scottish Gaelic still Alba), abhainn (earlier: abha), Manainn (earlier: Mana). Etymologically speaking, "n-stems" (comp. lat. natio-nationis)
• many of the nouns with the genitive ending in -n, -d form the dative through palatalisation of the genitive (-in,-id)
  e.g. fiche - fichead (gen.) - fichid (dat.), cara - carad (gen.) - caraid (dat.)

nom. plural:

• genitive form + -a : cathair - cathrach - cathracha, comharsan - comharsana,
• exceptions: athair - athar - aithreacha, abhainn - abhann - aibhneacha

gen. plural:

• genitive plural = nominative plural (mostly)
• genitive plural = genitive singular (some with the gen. sg. ending in -n, -d, less commonly ending in -ch, but mostly = nominative plural)

"regular" declension

examples: