First Declension

The first declension is one of five noun classes in Irish. It contains exclusively masculine nouns that end in a broad consonant in their basic form, the nominative singular. This is a very common and regular pattern, so learning its rules is a key step in mastering Irish grammar.

Nouns in this group include:

• Masculine nouns ending in a broad consonant.
• Many common nouns ending in -ach and -adh (but not verbal nouns in -adh).
• Many masculine first names, such as Seán, Séamas, and Pól.

Genitive Singular

To form the genitive singular, the final broad consonant is made slender. This process is called palatalisation or caolú.

• Adding -i-: For most nouns, you simply insert an i before the final consonant.
  • hata Sheáin - Seán’s hat
• Ending -(e)ach: This changes to -(a)igh. Note the spelling change from ch to gh.
  • capall an mharcaigh - the rider’s horse
• Ending -(e)adh: This changes to -(a)idh.
  • tús an tsamhraidh - the beginning of the summer

The pronunciation of the -(a)idh ending varies by dialect.

Some nouns require more significant vowel changes. The common change from ia to é does not apply to Brian (Briain), fiach (fiaigh), rian (riain), srian (sriain), trian (triain), cliant (cliaint), fiar (fiair), or giar (giair). These simply add an i.

Vocative Singular

The vocative singular, used for direct address, has the same form as the genitive singular. It is always preceded by the particle a, which causes lenition.

• A fhir! - Man!
• A Sheáin! - Seán!

Nominative Plural

Nouns in the first declension can have either a “weak” or a “strong” plural. The distinction matters for forming the genitive plural.

Weak Plurals

Weak plurals are formed in one of two ways:

1. Palatalisation: The plural form is identical to the genitive singular.
   • cat (a cat) -> cait (cats)
2. Ending -a: The ending -a is added to the nominative singular form.
   • cleas (a trick) -> cleasa (tricks)

Strong Plurals

Strong plurals are formed with a variety of endings.

• Ending -aí: This is common for nouns ending in -ach or -adh.
  • bealach (a way) -> bealaí (ways)
  • cogadh (a war) -> cogaí (wars)
• Endings -ta or -tha: These are often found with monosyllabic nouns ending in l, n, or r after a long vowel.
  • saol (a world, life) -> saolta (worlds, lives)
  • scéal (a story) -> scéalta (stories)
• Syncope and -e: Some polysyllabic nouns drop the vowel in the last syllable (syncope) and add -e.
  • solas (a light) -> soilse (lights)
  • uasal (a noble) -> uaisle (nobles)
• Other Endings: Less common endings include -anna and -acha.
  • bás (a death) -> básanna (deaths)

Genitive Plural

The form of the genitive plural depends on whether the nominative plural is weak or strong.

• For weak plurals, the genitive plural is the same as the nominative singular.
  • cosa na gcat - the legs of the cats (gcat is the eclipsed form of cat)
• For strong plurals, the genitive plural is the same as the nominative plural.
  • stair na gcogaí - the history of the wars (gcogaí is the eclipsed form of cogaí)

Vocative Plural

The vocative plural always takes the ending -a and is preceded by the particle a.

• A fheara! - Men!

Summary of Patterns

Core Patterns

This table summarises the most common “weak plural” patterns.

Common Endings

This table shows how common endings change across the different cases.

Examples