Caibidil a Dó: The Adjective (an Aidiacht)

Descriptive adjectives can be used predicatively (Tá an bhean mór = The woman is big) or attributively (an bhean mhór = the big woman). Besides that, one can turn them into adverbs (Tá an bhean ag obair go maith = The woman works well)

the predicative adjective (an aidiacht fhaisnéiseach)

Tá an fear mór - The man is big.
Tá an bhean mór - The woman is big.
Tá na fir mór - The men are big.
Tá na mná mór - The women are big.

Tá an fear mór - The man is big.
Is mór é an fear - The man is big.

Tá an fear go maith - The man is good.
Tá an cailín go hálainn - The girl is beautiful.
Tá an lá go deas - The day is nice.
Tá an aimsir go dona - The weather is bad.
Tá an samhradh go haoibhinn - The summer is splendid.
Tá an rós go breá - The rose is pretty.
Tá sé go holc - It is evil.
Tá sí go hiontach - She is wonderful.

Bíonn beoir maith i gcónaí - Beer is always good.
Bhíodh sí riamh álainn - She was always beautiful.
Tá an lá réasúnta deas. - The day is reasonably nice.
Bhí an aimsir sách dona inné - The weather was rather bad yesterday.

Tá an lá go deas - The day is nice.
Is deas é an lá - The day is nice.

Tá sé ag éirí mór - He is becoming big.
Éireoidh mé go maith - I will become good.
Má éiríonn tú sean - If you become old.
D'éirigh sí go hálainn as sin amach = She became beautiful since then.

One may also use other verbs in conjunction with predicative adjectives, (e.g. déan = do ):

Rinne sé an scian géar = He made the knife sharp.
Rinne sé na sceana géar = He made the knives sharp.

Not the difference to the attributive use! (adj. is then flexible, e.g. lenited after fem. noun: ghéar)
Rinne sé an scian ghéar = He made the sharp knife.
Rinne sé na sceana géara = He made the sharp knives.

Some additional adjectives can only be used predicatively, and not attributively (e.g. féidir = possible, fiú = worth, iomaí = many . These also only appear with the copula, not with bí or other verbs. e.g. Is féidir é sin = that is possible; Is fiú é = it is worth it

the attributive adjective (an aidiacht aitreabúideach)

Attributively used adjectives agree with the noun in case, number and gender.
Depending on the gender, case, number, they are declined and, if necessary, lenited.
They always come after the noun being modified (as opposed to German or English).
e.g.:

the big man = an fear mór,
of the big man = an fhir mhóir,
the big woman = an bhean mhór,
of the big woman = na mná móire,
the big women = na mná móra.

• after feminine personal names:
  mac Bhríd bheag = the son of small Bríd but mac na mná bige = the son of the small woman
• after adverbs:
  mac na mná réasúnta beag = the son of the reasonably small woman but mac na mná bige = the son of the small woman

• after numbers 2-10 and after beirt: trí bhád mhóra = three big boats (bhád = singular, mhóra = plural!)

• in the form of prefixes: e.g.: the bad man = an drochfhear
• certain pronominal adjectives: e.g.:  gach fear = every man, gach uile bhean = every woman (see below)

Order of attributes:

• after the noun, the closest descriptive adjectives come first: an bhean mhaith = the good woman.
• pronominal adjectives follow after that: an bhean mhaith seo eile = this other good woman (or: an bhean mhaith eile seo).
• also substantivised genitive attributes come after descriptive adjectives: bean mhaith na scoile = the good woman of the school, scian mhór phoca = big pocketknife.

adjectives als adverb (aidiachtaí mar dhobhriathar)

formation:

e.g.:
Tá mé sciobtha - I am fast. (predicative)
D'oibrigh mé go sciobtha - I work fast. (adverbial)

exceptions

• go is omitted in the case of an additional adverb  e.g.: go dona = bad,  réasúnta dona = reasonably bad.
• go does not appear if a copular clause is used instead of the adverb. e.g.: Is cúramach a rinne sé é = Carefully, he did it
  (colloquially there is a remnant h-prefix: e.g. instead of Rinne sé go hiontach é > Is (h)iontach a rinne sé é = Amazingly, he did it )
• instead of go one uses níos preceding the adverbial comparative form. e.g.: Rinne sé níos cúramaí é = He did it more carefully

use:

• adverbial use of an adjective:  Bionn Bríd ag obair go cúramach = Bríd usually works carefully.
• predicative (!) Use of some subjective evaluatory adjectives is done with the verb bí:  Tá an teach go dona = the house is bad.

pronominal adjectives (aidiachtaí forainmneacha)

Pronominal adjectives do not modify a noun, but imply a classification, closer division or (oppositely) a generalisation.
Principally, the article is an example of a pronominal adjective. Also numbers, possessive pronouns, demonstrative pronouns and interrogatives are used as pronminal adjectives. On the other hand, many pronominal adjectives serve together with general nouns in place of a substantivised pronoun (e.g.: duine éigin = some person > somebody ).
Some adjectives like many, such are replaced by nouns. See also indefinite pronouns

Pronominal adjectives are always used attributively. They are not declined and mostly also not lenited (though eclipsis occurs often). Some of these adjectives come before (e.g. gach fear = every man), some after the noun(e.g. fear éigin = any man)

• gach = every. e.g.: gach fear = every man; also:
  gach uile = every. e.g.: gach uile fhear = every man (chuile in Connacht, ach uile in Ulster)
  gach aon = every. e.g. gach aon fhear = every man (chaon in Connacht, achan in Ulster)
• uile (requires lenition)(in Ulster, Connacht uilig)
  = all, the whole (precedes noun in the plural, with an article or a possessive pronoun) e.g. na huile dhaoine = all people, all of the people
  = all, every (precedes the noun in the singular, with an article or a possessive pronoun) e.g. an uile mhaitheas = the whole goodness, d'uile mhaitheas = all your goodness, all of your goodness
  = whole (after noun in the singular) e.g. an scéal uile = the whole story
  = all (after the noun in the plural or the pronoun) e.g. sinn uile = we all
• éigin = someone. e.g. fear éigin = some man (in Connacht éicint, icínt, in Ulster inteach, ínneach, among others, shared original form is most likely éiginteach)
• aon, ar bith = some, any (in questions), no, none (in neg. statements) e.g. aon fhear, fear ar bith = any man, no man
• cíbé, pé (ar bith) = whatever, whichever. e.g. cibé duine = whatever person, pé áit = wherever
• eile = other, another. e.g. fear eile = another man, rud eile? = something else?
• céanna = same, very e.g. an fear céanna = the same man
• araile = certain. e.g. lá araile = on a certain day
• áirithe = certain. e.g. lá áirithe = on a certain day (in Connacht, Ulster áithrid)

[1] : Once it was the rule of thumb that only adverbs could appear with the verb bí, always. ("adverbs with bí, adjectives with is")
Today this only is the case for a few adjectives (see above), while the predicative use of adjectives with bí is possible and has become common.