Declension

In Irish, adjectives that describe a noun usually come after it. These adjectives must change their form to “agree” with the noun in gender, number, and case. This process is called declension. While this might seem complex, understanding the patterns will make your Irish sound much more natural.

When to Decline Adjectives

In Standard Irish, only descriptive adjectives that follow a noun (attributive adjectives) are declined. Adjectives that come before a noun or are used predicatively (after a verb like bí) do not change their form.

• Attributive (declined): an fear mór (the big man)
• Predicative (not declined): Tá an fear mór. (The man is big.)
• Preceding a noun (not declined): an seanfhear (the old man)

The comparative form of an adjective is always used predicatively and is therefore never declined.

Mutation Rules for Adjectives

Adjectives are lenited in several specific grammatical contexts to ensure smooth pronunciation and flow.

Lenition of an adjective occurs:

• After a feminine singular noun in the nominative case.
  • bean mhór - a big woman
• After a masculine singular noun in the genitive case.
  • hata an fhir mhóir - the hat of the big man
• After a singular noun in the vocative case (both masculine and feminine).
  • A fhir bhig! - Oh, little man!
  • A bhean bheag! - Oh, little woman!
  • Note that the adjective is not lenited in the vocative plural: A fheara beaga! (Oh, little men!)
• After a plural noun ending in a slender consonant (weak plurals only).
  • fir mhóra - big men
• After a noun in the dative singular, following a preposition and the definite article.
  • don tsráid mhór - to the big street (feminine)
  • ag an mbean mhór - with the big woman (feminine)

Agreement in Case, Gender, and Number

The form of an adjective changes depending on the noun it follows. The main differences appear in the genitive, vocative, and dative cases.

Gender

In the nominative singular, the only difference between masculine and feminine is that an adjective following a feminine noun is lenited.

• an fear mór - the big man (masculine)
• an bhean mhór - the big woman (feminine)

More significant form changes occur in the genitive singular.

Genitive Case

The way an adjective forms its genitive singular determines which of the three main declension classes it belongs to.

• First Declension: The adjective’s final consonant is made slender (palatalised).
• Second Declension: The adjective’s final consonant is made broad (depalatalised) and an -a is added.
• Third Declension: The adjective (ending in a vowel) does not change.

The genitive plural of an adjective depends on the noun’s plural type:

• With a weak plural noun, the adjective uses its nominative singular form.
  • na bhfear mór - of the big men
• With a strong plural noun, the adjective uses its nominative plural form.
  • na mbuachaillí móra - of the big boys

Vocative Case

In the vocative singular:

• After a masculine noun, the adjective takes its genitive singular form.
  • A fhir mhóir! - Oh, big man!
• After a feminine noun, the adjective takes its nominative singular form.
  • A bhean mhór! - Oh, big woman!

In the vocative plural, the adjective always takes its nominative plural form.

• A fheara móra! - Oh, big men!

Dative Case

In the dative singular:

• First Declension adjectives are palatalised.
  • ag an bhfuinneoig bhig - at the small window
• Second and Third Declension adjectives are unchanged.
  • ag an bhfuinneoig bhuí - at the yellow window

The dative plural is always the same as the nominative plural.

• ag na fuinneoga beaga - at the small windows

First Declension

The first declension is the largest group, containing most adjectives that end in a consonant.

Subgroup 1: Adjectives ending in a broad consonant

This group includes most adjectives ending in a broad consonant, such as mór (big), dearg (red), and gorm (blue).

Genitive Singular

• After a masculine noun, the final consonant is palatalised (made slender). For example, mór becomes mhóir.
• After a feminine noun, the final consonant is palatalised and an -e is added. For example, mór becomes móire.

Nominative Plural The plural is formed by adding -a. For example, mór becomes móra. Syncope can also occur here: ramhar becomes ramhra.

Examples with mór (big)

With a weak plural noun (fear/bean)

With a strong plural noun (teach/muir)

Subgroup 2: Adjectives ending in a slender consonant

This group includes all adjectives ending in a slender consonant, such as maith (good) and álainn (beautiful).

Genitive Singular

• After a masculine noun, the adjective does not change form (as it is already slender), but it is lenited. For example, maith becomes mhaith.
• After a feminine noun, an -e is added. For example, maith becomes maithe.

Nominative Plural The plural is formed by adding -e, making it identical to the feminine genitive singular. For example, maith becomes maithe.

Examples with maith (good)

With a weak plural noun (fear/bean)

With a strong plural noun (dalta/ainnir)

Subgroup 3: Adjectives resistant to palatalisation

A specific set of adjectives ending in a broad consonant do not become slender in the masculine genitive singular. These include adjectives ending in -ll, -nn, -rr, or a long vowel + -ch (e.g., -íoch, -ách).

Examples include fann (weak), gann (scarce), imníoch (anxious), fliuch (wet), and beacht (precise). This group also includes amh (raw), cúng (narrow), deas (nice), tearc (scarce), trom (heavy), and tur (dry).

Genitive Singular

• After a masculine noun, the adjective does not change. For example, fann becomes fhann.
• After a feminine noun, it behaves like Subgroup 1: the final consonant is palatalised and -e is added. For example, fann becomes fainne. The adjective mall (slow) has a slightly irregular feminine genitive: moille.

Nominative Plural The plural is formed by adding -a. For example, fann becomes fanna.

Examples with mall (slow)

Examples with imníoch (anxious)

Subgroup 4: Adjectives ending in -ach

This group includes all polysyllabic adjectives ending in -ach or -each, such as salach (dirty), Éireannach (Irish), and díreach (direct).

Genitive Singular

• After a masculine noun, the ending is palatalised to -aigh. For example, salach becomes shalaigh.
• After a feminine noun, the ending becomes -aí. For example, salach becomes salaí.

Nominative Plural The plural is formed by adding -a, becoming -acha. For example, salach becomes salacha.

Examples with salach (dirty)

With a weak plural noun (fear/girseach)

With a strong plural noun (dalta/ainnir)

Second Declension

The second declension covers adjectives ending in -úil and some ending in -ir. These adjectives behave similarly to third-declension nouns.

Subgroup 1: Adjectives ending in -úil

This common ending is used to form adjectives from nouns, similar to “-ful” or “-like” in English. Examples include bródúil (proud) and cúramach (careful).

Genitive Singular

• After a masculine noun, the form is unchanged. For example, bródúil becomes bhródúil.
• After a feminine noun, the ending is depalatalised (made broad) and -a is added, becoming -úla. For example, bródúil becomes bródúla.

Nominative Plural The plural form is the same as the feminine genitive singular: -úla.

Examples with bródúil (proud)

With a weak plural noun (fear/bean)

With a strong plural noun (dalta/ainnir)

Subgroup 2: Adjectives ending in -ir

This small group includes adjectives like deacair (difficult), cóir (just), and socair (calm).

Genitive Singular

• After a masculine noun, the form is unchanged. For example, deacair becomes dheacair.
• After a feminine noun, the ending is depalatalised and undergoes syncope, adding -a to become -ra. For example, deacair becomes deacra.

Nominative Plural The plural form is the same as the feminine genitive singular: -ra.

Examples with deacair (difficult)

With a weak plural noun (úll/bróg)

With a strong plural noun (rud/troid)

Third Declension

The third declension is the simplest, covering all adjectives that end in a vowel, such as crua (hard), buí (yellow), and dána (bold).

These adjectives are indeclinable; they do not change their form for case, gender, or number. They are still subject to the standard mutation rules (e.g., lenition after a feminine singular noun).

Examples with crua (hard)

Irregular Adjectives

A small number of common adjectives have irregular declension patterns.