A sign in the Irish language which displays the word Caisleán with initial mutation.

Irish, like all modern Celtic languages, is characterized by its initial consonant mutations.^[1] 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 Manx and colloquial Scottish Gaelic, uses two mutations on consonants: lenition (Irish: séimhiú [ˈʃeː.vʲuː]) and eclipsis (urú [ˈʊ.ɾˠuː]) (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.

There are also two mutations, t-prothesis and h-prothesis, found on vowel-initial words.

See Irish phonology for a discussion of the symbols used on this page.

Summary table

This table shows the orthographical and phonological effects of both lenition and eclipses, as well as h-prothesis and t-prothesis. In this table a given vowel is represented by ⟨v⟩ and /@/. An prothesised consonant is broad before ⟨a, á, o, ó, u, ú⟩ and slender before ⟨e, é, i, í⟩.

Environments of Lenition

After proclitics

After the definite article

The definite article triggers lenition of:

1. a feminine noun in the nominative singular

   an bhean "the woman"

2. a masculine noun in the genitive singular

   an fhir "of the man" e.g. carr an fhir, the man's car (car of the man)

3. a noun in the dative singular, when the article follows one of the prepositions de "from", do "to" or i "in"

   do + an = don: don fhear "to the man"

   de + an = den: den bhean "from the woman"

   i + an = sa(n): sa chrann "in the tree"; san fhómhar "in the autumn"

• /d̪ˠ, dʲ/ and /t̪ˠ, tʲ/ are never lenited after the article:

an deoch "the drink", although deoch is feminine nominative singular

an tí "of the house", although tí is masculine genitive singular

• Where an /sˠ, ʃ/ would be lenited after the article, it becomes /t̪ˠ, tʲ/ (rather than /h/), written ts:

an tsúil /ən̪ˠ t̪ˠuːlʲ/ "the eye" (fem. nom. sg.)

an tsaoil /ən̪ˠ t̪ˠiːlʲ/ "of the world" (masc. gen. sg.)

After the vocative particle a

• a Bhríd "Bríd!"
• a Sheáin "Seán!"
• a chairde "my friends!"

After possessive pronouns

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

• mo mhac "my son"
• do theach "your house"
• a pheann "his pen"

After certain prepositions

• de chrann "out of a tree"
• faoi chrann "under a tree"
• mar dhuine "as a person"
• ó Chorcaigh "from Cork"
• roimh mhaidin "before morning"
• trí shioc agus shneachta "through frost and snow"
• um Cháisc "at Easter"
• idir fhir agus mhná "both men and women"
• ar bhord "on a table"

After the preterite/conditional of the copula

• Ba dhuine mór é. "He was a big person."
• Ba dheas uait é. "That was nice of you."

After the preterite preverbal particles

• Níor mhúinteoir é. "He was not a teacher."
• Níor thug mé "I didn't give"
• Ar shagart é? "Was he a priest?"
• Ar tháinig sé? "Did he come?"

After certain preverbal particles

• ní thuigim "I don't understand"
• má thagann sé "if he comes"
• an fear a thabharfaidh dom é "the man who will give it to me"

A verb in the preterite, imperfect or conditional

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

• bhris mé "I broke"
• bhrisinn "I used to break"
• bhrisfinn "I would break"

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:

• aon bhó amháin "one cow"
• an chéad bhliain "the first year"
• dhá theach "two houses"
• beirt fhear "two men"
• trí bhád "three boats"
• ceithre bhó "four cows"
• cúig phunt "five pounds"
• sé mhí "six months"

After preposed adjectives

Constructions of adjective + noun are written as compounds.

• seanbhean "old woman"
• drochdhuine "bad person"
• dea-sheirbhís "good deed"
• nuatheanga "modern language"
• tréanmhuir "stormy sea"
• fíorchneas "true skin"
• ardbhrú "high pressure"
• ógfhear "young man"

After most prefixes

• an-bheag "very small"
• ró-bheag "too small"
• aisghabháil "retake"
• athbhliain "new year"
• dobhréagnaithe "undeniable"
• fochupán "saucer"
• forbhríste "overalls"
• idirchreidmheach "interconfessional"
• ilphósadh "polygamy"
• leasmháthair "stepmother"
• míshásta "unhappy"
• neamhchodladh "insomnia"
• príomhchathair "capital city"
• sobhriste "fragile"

The second part of a compound

• ainmfhocal "noun" (lit. "name word")
• dúghorm "dark blue"
• státfhiach "national debt"

In head + modifier constructions

In these constructions coronals are lenited even following other

• aimsir bháistí "rainy weather" (lenition after a feminine singular noun)
• buidéil shú "bottles of juice" (lenition after a plural ending in a slender consonant)
• teach Sheáin "Seán's house" (lenition of a definite noun in the genitive)

Postposed adjectives in certain circumstances

• bean dheas "a pretty woman" (lenition after a feminine singular noun)
• na fir mhóra "the big men" (lenition after a plural noun ending in a slender consonant)
• ainm an fhir bhig "the name of the small man" (lenition after a masculine singular noun in the genitive)
• sa chrann mhór "in the big tree" (lenition after a noun lenited by virtue of being in the dative after den, don, or sa(n))

Environments of Eclipsis

Eclipsis on a sign in Tramore: Fánán na mBád means "slip of the boats." Even in an all-caps inscription, the eclipsis letter is not capitalised.

Eclipsis on a sign in Raphoe: Sráith na nGael means "Row of the Gaels."

After plural possessive pronouns

The possessive pronouns that trigger eclipsis are ár "our", bhur "your (pl.)", a "their"

• ár gcairde "our friends"
• bhur bpáistí "your (pl.) children"
• a mbád "their boat",

Note that 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:

• seacht gcapall "seven horses"
• ocht n-asal "eight donkeys"
• naoi gcat "nine cats"
• deich bpeann "ten pens"

After the preposition i "in"

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

• i dteach "in a house"
• in Éirinn "in Ireland"

Genitive plural nouns after the definite article

The genitive plural article na eclipses a following noun:

• na n-asal "of the donkeys"
• na bhfocal "of the words"

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)

• ag an bhfear "by the man"
• ar an gcrann "on the tree"

After certain preverbal particles

• an poll a dtagann na coiníní as "the hole that the rabbits come out of"
• An dtagann sé gach lá? "Does he come every day?"
• Cá bhfuil mo spéaclaí? "Where are my glasses?"
• Dúirt sé go dtiocfadh sé. "He said that he would come."
• dá mbeadh a fhios sin agam "if I had known that"

Changes to vowel-initial words

A vowel-initial word does not change if a lenition is expected:

• an oíche "the night" (feminine singular nominative noun after definite article)
• an uisce "of the water" (masculine singular genitive noun after definite article)
• ó Albain "from Scotland" (noun after leniting preposition)
• seanathair "grandfather" (noun after preposed adjective: sean "old" + athair "father")

However, if neither eclipsis nor lenition is expected, an initial vowel may acquire a prothetic onset consonant. For example, a vowel-initial masculine singular nominative noun requires a t- (a voiceless coronal plosive) after the definite article:

• an t-uisce "the water" (masculine singular nominative)

Otherwise, there is the prothetic onset h (a voiceless glottal fricative), which comes only when both the following conditions are met:

1. a proclitic causes neither lenition nor eclipsis of consonants.
2. a proclitic itself ends in a vowel.

Examples of h-prothesis

• a haois "her age" (after possessive pronoun a "her"; compare with a aois, "his age" and a n-aois, "their age" with regular urú)
• go hÉirinn "to Ireland" (after preposition go "to, towards")
• le hAntaine "with Antaine" (after preposition le "with")
• na hoíche "of the night" (on feminine singular genitive noun after definite article)
• na héin "the birds" (on plural nominative/dative noun after definite article)
• chomh hard le caisleán "as high as a castle" (after chomh [xo] "as")
• go hálainn "beautifully" (after adverb-forming particle go)
• Ná himigh uaim "Don't leave me!" (after negative imperative particle ná "don't")
• an dara háit "the second place" (after an ordinal numeral)