Irish initial mutations

Originally these mutations were phonologically governed external sandhi effects: lenition was caused by a consonant being between two vowels, and eclipsis when a nasal preceded an obstruent, including at the beginning of a word.

Lenition caused stops and *m to become fricatives, *s to debuccalise to [h], *f to elide, and the liquids *l, *n, *r to split into fortis and lenis variants.

For example Proto-Celtic *knāmis → cnáim → cnáimh "bone", and *abalnā → aball → abhaill "apple tree".

The prosthetic ⟨t⟩- of vowel initial words is a fossilised fragment of the Proto-Celtic masculine definite article *sindos.

For example, the Proto-Celtic genitive plural of the definite article *sindoisom has lost its final nasal and been reduced to na but it now causes the eclipsis of a following consonant or the prothesis of ⟨n-⟩ to a vowel.

See also Irish orthography which has a table showing non-initial lenited consonants which elided or vocalised to form diphthongs or long vowels.

The singular form is used after numbers and is lenited in the following cases: Constructions of adjective + noun are written as compounds.

The genitive plural article na eclipses a following noun: In western and southern dialects, nouns beginning with a noncoronal consonant are eclipsed after combinations of preposition + article in the singular (except den, don, and sa(n), which trigger lenition) In environments where lenition occurs a vowel initial word remains unchanged: However, In environments where neither eclipsis nor lenition is expected, an initial vowel may acquire a prothetic consonant.