Caibidil a Dó: the adjective (an Aidiacht)

Which adjectives are declined :
Descriptive attributive adjectives, that follow a noun.
Preceding a noun, adjectives are not declined .
Also predicative adjectives are not declined .
Therefore, the comparative is not declinable.

lenition rules:
lenition of an adjective is used:

1. after feminine nouns in the nominative/accusative singular e.g.: bean mhór = big woman
2. after nouns in the dative after preposition + article + noun, if
   • the noun is lenited (always after feminine nouns, but not after masculine nouns in Connacht) e.g. don tsráid mhór = to the main street, don fhear m(h)ór = to the big man
   • the noun is eclipsed (always after feminine nouns, but not after masculine nouns in Connacht) e.g.: ag an mbean mhór = at the big woman, ag an bhfear m(h)ór = at the big man
3. after masculine nouns in the genitive singular e.g.: an fhir mhóir = of the big man
4. after femin. and masc. nouns in the vocative singular e.g a fhir bhig = little man!, a bhean bheag = little woman!
   but not in the plural: e.g.: a fheara beaga = little men!
5. after femin. and masc. nouns in the nominative plural, that end in slender consonants (only possible in the weak plural) e.g.: fir mhóra = big men
6. the "dentals"-rule is not used with attributive adjectives (so d,t,s are indeed lenited also after nouns ending in d,n,t,l,s) e.g.: an bhean dheas = the nice woman

genitive plural:
The genitive plural is equivalent to:

1. the nominative singular (by a weak plural of the noun) e.g. na bhfear mór = the big men/the big men's
2. the nominative plural (by a strong plural of the noun) e.g: na mbuachaillí móra = the big boys'

1. after masculine nouns (all declensions!) the genitive singular e.g.: a fhir mhóir = big man!, a chara mhóir = big friend! (but not if inanimate objects are used as nicknames for people, and also not after collectives, while then not the vocative but the nominative takes hold)
2. after feminine nouns of the nominative singular e.g.: a bhean mhór = big woman!

1. the nominative plural e.g.: a fheara móra = big men!

• formed in the 1st declension via palatalisation (e.g. beag - big);
  e.g.: ag an bhfuinneoig bhig = at the small window, don mhnaoi mhóir = to the big woman
• in the 2nd and 3rd declensions, the adjective remains unchanged.
  e.g.: ag an bhfuinneoig bhuí = at the yellow window

The dative plural is always the same as the nominative plural.
e.g.: ag na fuinneoga beaga = at the small windows
Even if one uses the old dative plural suffix -ibh with a noun, it is never used with an adjective:
e.g.: ag na fuinneogaibh beaga = at the small windows

All adjectives, ending in consonants.
With the exception of those ending in auf -úil or -ir (2nd declension)
Generally, these adjectives are declined like the nouns of the 1st declension (masculine) or the 2nd declension (feminine).

1st subgroup: adjectives, ending in a broad consonant

Who belongs here:
Most of the adjectives that end in a broad consonant.
(slender consonant: 2nd subgroup)
In written Irish, this means that the last vowel is a a, o or u
What doesn't belong are adjectives ending in -ll,-nn (3rd subgroup) or -ch (3rd and 4th subgroup)
examples: mór = big, dearg = red, gorm = blue, olc = evil

forming the genitive in the singular:

• After a masculine noun via palatalisation, the word ends then in a slender consonants (in written Irish, that makes the last vowel in the word either e or i).
  e.g.: mór - móir
• After feminine nouns also leads to palatalisation and additionally a suffix of -e
  e.g.: mór - móire
  Polysyllabic adjectives tend to shorten (syncopation) when additional suffixes are added, if possible
  e.g. ramhar - raimhre (instead of "*ramhaire")

nominative plural

• The plural is formed by adding the suffix -a to the singular form.
  e.g.: mór - móra
  Polysyllabic adjectives  act as in the genitive w.r.t. syncopation
  e.g.: ramhar - ramhra (instead of "*ramhara")

Paradigms
weak plural

examples
mór = big, after a noun with a weak plural

2nd subgroup: adjectives ending in slender consonants

Who belongs here:
All adjectives ending in a slender consonant.
In written Irish, that means the last vowel is e or i
examples: maith = good, áillinn = beautiful, etc.

forming the genitive in the singular

• palatalisation is not necessary, because the word alreasy ends in a slender consonant.
• After masculine nouns, the adjective then has the same form as in the nominative singular (but lenited).
  e.g.: maith - mhaith
• After feminine nouns an -e is added to the end.
  e.g.: maith - maithe
  polysyllabic verbs tend to syncopate
  e.g. saibhir - saibhre (instead of "*saibhire")
  e.g. domhain - doimhne (instead of "*domhaine")

nominative plural

• The plural is formed by adding the suffix -e to the singular form (making it look the same as the genitive singular feminine).
  e.g. maith - maithe
  polysyllabic verbs  also here tend to syncopate
  e.g. saibhir - saibhre (instead of "*saibhire")
  e.g. aoibhinn - aoibhne (instead of "*aoibhinne")

Paradigms
weak plural

examples
maith = good, after a noun with a weak plural

3rd subgroup: adjectives ending in -ll,-nn or long vowel + -ch

Who belongs here:
Some adjectives, ending in certain broad consonants, are not palatalised in the masculine genitive.
This includes the adjectives ending in -ll,-nn or a long vowel + -ch (e.g.-íoch, -ách, -óch -iuch, -eoch etc.)
As well as adjectives ending in a broad -cht, -ng, -m, and some ending in -r/-rr
examples:  fann = weak, fionn = blond, dall = blind, imníoch = anxious, gleoch = loud, fliuch = wet, bocht = poor
adjectives ending in a short -ach do not belong here (4th subgroup)

forming the genitive in the singular

• After masculine nouns, (as opposed to the 1st subgroup) the genitive is the same as nominative singular , that means no palatalisation, despite theoretical possibility (wrong: fann > "*fainn", imníoch > "*ímních").
  e.g.: fann - fhann, imníoch - imníoch
• After feminine nouns, the adjective always palatalised and an -e is added to the end.
  e.g.: fann - fainne; fionn - finne, imníoch - imníche

nominative plural

• The plural is formed by adding the suffix -a to the singular form.
  e.g. fann - fanna, fionn - fionna, imníoch - imníocha

Paradigma
weak plural

examples
fionn = blond, after a weak plural

4th subgroup: adjectives ending in -ach

Who belongs here:
All adjectives, that end in -ach (or -each).
But not adjectives ending in -och, -ách, -óch, -íoch (3rd subgroup)
examples: salach = dirty, Éireannach = Irish, díreach = direct etc.

forming the genitive in the singular

• this time, even the masculine nouns incur palatalisation,
  but the palatalised form is not -aich but -aigh
  e.g.: salach - salaigh
• After feminine nouns, the adjective is palatalised as well and an -e is added, but instead of "-aighe" one writes -aí
  e.g.: salach - salaí

nominative plural

• The plural is formed by adding the suffix -a to the singular form
  e.g. salach - salacha

Paradigma
weak plural

examples
salach = dirty,  after a noun with a weak plural

Who belongs here:
All adjectives ending -úil (1st subgroup) and some ending in -ir (2nd subgroup)
Basically, these adjectives are declined like the nouns of the 3rd declension.

1st subgroup: adjectives ending in -úil

The suffix -úil stems from the word (s)amhail = same, the same and is equivalent to the German suffix -lich or the English -like, -ly, related to the latin simul-. Through this samhail also the definite feminine genitive (samhail - samhla).
Prior to the spelling reform, this suffix was also written -amhail (bródamhail > bródúil)

forming the genitive in the singular

• The masculine form is the same as nominative singular
  e.g. bródúil - bródúil
• As opposed to the 1st declension, follwing feminine nouns depalatalisation occure and the suffix -a is added,
  e.g. bródúil - bródúla

nominative plural

• The plural is as well formed via depalatalisation and the addition of the suffix -a to the singular form (looks the same as the feminine genitive singular)
  e.g. bródúil - bródúla

Paradigma
weak plural

examples
bródúil = proud , after a noun with a weak plural

2nd subgroup adjectives ending in -ir

Who belongs here:
some adjectives that end in -ir (mostly -air)
examples: deacair = difficult; cóir = just, socair = quiet

forming the genitive in the singular

• The masculine form is the same as the nominative singular .
  e.g.: deacair - deacair
• Feminine form: depalatalisation, as well as the abbreviation of the 2nd syllable (syncopation) + -a.
  e.g.: deacair - deacra

nominative plural

• The plural ist in der form is the same as feminine genitive singular
  e.g.: deacair - deacra

Widely similar to the 1st subgroup

Paradigma
weak plural

examples
deacair = difficult, after a noun with a weak plural

Who belongs here:
All adjectives ending in a vowel.

forming the genitive in the singular

• The form is equivalent to masculine and feminine of the nominative singular
  e.g. crua - crua

nominative plural

• The form is equivalent to the nominative singular as well
  e.g. crua - crua

These adjectives do not change at all (aside from an occasional lenition)

Paradigms
weak plural

examples
crua = hard

A few adjectives have irregular declension forms.
(irregular forms highlighted in red)