Irish, like all modern Celtic languages, is characterised by its initial consonant mutations. These mutations affect the initial consonant of a word under specific morphological and syntactic conditions. The mutations are an important tool in understanding the relationship between two words and can differentiate various meanings. Irish, like Scottish Gaelic and Manx, features two initial consonant mutations: lenition and eclipsis (urú ) (the alternative names, aspiration for lenition and nasalisation for eclipsis, are also used, but those terms are misleading). 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. Irish also features t-prothesis and h-prothesis, related phenomena which affect vowel-initial words. See Irish phonology for a discussion of the symbols used on this page.

Historical development

Lenition

Lenition as an initial mutation originally stems from the historical allophonic lenition of an intervocalic consonant, both word internally and across word boundaries, i.e if a word ended in a vowel and the next word began with a consonant + a vowel, the consonant lenited. Today, these former final vowels are usually elided, but the lenition of following consonants remains and has been grammaticised. For example, Proto-Celtic *esyo "his" caused the lenition of a following consonant due to its final vowel and its modern form a now causes lenition, keeping it distinct from a "her" and a "their", which cause h-prothesis and eclipsis respectively. Lenition caused stops and *m to become fricatives, *s to debuccalise to, *f to elide, and the liquids *l, *n, *r to split into fortis and lenis variants. Though by the end of the Middle Irish period lenited *m largely lost its nasal quality, lenited *t debuccalised to, and lenited *d lost its coronal articulation. Lenition did not only occur word initially, though non-initial lenition was never grammaticised. For example Proto-Celtic *knāmis → cnáim → cnáimh "bone", and *abalnā → aball → abhaill "apple tree".

Prothetic - and -

While it is not initially apparent, the prothesis of and stems from historical lenition combined with vowel reduction. The prosthetic - of vowel initial words is a fossilised fragment of the Proto-Celtic masculine definite article *sindos. Before vowels, the *s of the ending *-os was lenited to, which (combined with the loss of the *-o-) devoiced the preceding *-d- to *-t. The prosthetic of initial words is a fossilised fragment of the d of Proto-Celtic nominative feminine definite article *sindā and masculine genitive definite article *sindī. Since they ended in vowels, a following word initial *s was lenited to which (combined with the loss of the *-ā, *-ī) devoiced the preceding *-d to *-t. The prothetic of vowel initial words has two origins, the first being epenthetic to avoid vowel hiatus, and the second being the fossilised remnant of a historic consonant. For example, the *s of Proto-Celtic *esyās "her" was lenited between vowels to. Overtime *esyās was reduced to a but the remains when it is followed by a vowel initial word but is now written as part of the following word.

Eclipsis

Eclipsis originally stems from the historical coalescence of consonant clusters beginning with a nasal, both word internally and across word boundaries, i.e if a word ended in a nasal and the next word began with a stop or labial fricative, they would coalesce. Today, many of the former final nasals have been elided, but still have an effect on the pronunciation of a following consonant, which has been grammaticised. 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 to a vowel. The cluster reductions involved in eclipsis turned nasal stops followed by a voiced stop into nasal stops, nasal stops followed by a voiceless stop into voiced plosives, nasal stops followed by a voiceless labial fricative into a voiced fricative, and words which have lost their final nasal add an to vowel initial words. These cluster reductions did not only occur word initially, though non-initial coalescence was never grammaticised. For example, Proto-Celtic *lindos → lind → linn "pool", and *kʷenkʷe → cóic → cúig "five".

Summary table

This table shows the orthographical and phonological effects of lenition, eclipsis, h-prothesis, and t-prothesis. Vowels are represented by and. Consonants are broad before and slender before. See also Irish orthography which has a table showing non-initial lenited consonants which elided or vocalised to form diphthongs or long vowels. ***** Not all dialects contrast lenited and from their unlenited forms. See Irish Phonology.

Environments of Lenition

After proclitics

After the definite article

The definite article triggers the lenition of: Lenition is blocked when a coronal consonant is preceded by an. Instead of leniting to, after the definite article, become (written ):

After the vocative particle a

After possessive pronouns

The possessive pronouns that trigger lenition are mo "my", do "your (sg.)", a "his"

After certain prepositions

After the preterite/conditional of the copula

After the preterite preverbal particles

After certain preverbal particles

A verb in the preterite, imperfect or conditional

These were originally preceded by the particle do and often still are in Munster.

In modifier + head constructions

Lenition is blocked in these constructions if two coronals are adjacent.

After certain numbers

The singular form is used after numbers and is lenited in the following cases:

After preposed adjectives

Constructions of adjective + noun are written as compounds.

After most prefixes

The second part of a compound

In head + modifier constructions

In these constructions coronals are lenited even following other

Postposed adjectives in certain circumstances

Environments of Eclipsis

After plural possessive pronouns

The possessive pronouns that trigger eclipsis are ár "our", bhur "your (pl.)", a "their" a can mean "his", "her" or "their", but these different uses can still be distinguished, since a causes lenition when used as "his" (a bhád), causes eclipsis when used as "their" (a mbád), and neither when used as "her" (a bád).

After certain numbers

The numbers that trigger eclipsis (the noun being in the singular) are:

After the preposition i "in"

Before a vowel in is written instead of i n-.

Genitive plural nouns after the definite article

The genitive plural article na eclipses a following noun:

Dative singular nouns after the definite article

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)

After certain preverbal particles

Changes to vowel-initial words

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. For example, a vowel-initial masculine singular nominative noun requires a (a voiceless coronal plosive) after the definite article: Additionally, there is the prothetic (a voiceless glottal fricative), which occurs when both the following conditions are met: Examples of h-prothesis: