Irish, like all modern Celtic languages, is characterized 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.
The unmutated form, i.e. the "basic" form that dictionary entries are listed under, is known as the radical.
Effects of lenition 
Consonants change as follows when they undergo lenition:
|b /bˠ/, /bʲ/||bh /w/, /vʲ/|
|c /k/, /c/||ch /x/, /ç/|
|d /d̪ˠ/, /dʲ/||dh /ɣ/, /j/|
|f /fˠ/, /fʲ/||fh (silent)|
|g /ɡ/, /ɟ/||gh /ɣ/, /j/|
|m /mˠ/, /mʲ/||mh /w/, /vʲ/|
|p /pˠ/, /pʲ/||ph /fˠ/, /fʲ/|
|s /sˠ/, /ʃ/||sh /h/|
|Note: sc, sm, sp, and st do not mutate.|
|t /t̪ˠ/, /tʲ/||th /h/|
The other consonants do not change under lenition.
Environments of lenition 
After proclitics 
- After the definite article an
The definite article triggers lenition of:
- a feminine noun in the nominative singular
- an bhean "the woman"
- a masculine noun in the genitive singular
- an fhir "of the man"
- a noun in the dative singular, when the article follows one of the contractions den (“from the”), don (“to the”) or sa (“in the”) (san before a vowel sound) (note: in some dialects, lenition occurs also after other preposition + article combinations):
- d and t are never lenited after the article:
- Where s would be lenited after the article, it becomes ts /t̪ˠ, tʲ/ (rather than sh /h/):
- After the vocative particle a
- After possessive pronouns
- 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 is
- 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 (d, l, n, s, t) 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-shláinte "good health"
- 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, navy blue" (lit. "black-blue")
- státfhiach "national debt"
In head + modifier constructions 
In these constructions coronals are lenited even following other coronals.
- Genitive nouns in certain circumstances
- 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))
Effects of eclipsis 
1. Consonants change as follows when they undergo eclipsis. When eclipsed words are capitalized, it is the consonant of the radical that is capitalized, while the letter(s) showing eclipses remain small.
|Radical||Eclipsed||Capitalized radical||Capitalized eclipsed|
|b /bˠ/, /bʲ/||mb /mˠ/, /mʲ/||B||mB|
|c /k/, /c/||gc /ɡ/, /ɟ/||C||gC|
|d /d̪ˠ/, /dʲ/||nd /n̪ˠ/, /nʲ/||D||nD|
|f /fˠ/, /fʲ/||bhf /w/, /vʲ/||F||bhF|
|g /ɡ/, /ɟ/||ng /ŋ/, /ɲ/||G||nG|
|p /pˠ/, /pʲ/||bp /bˠ/, /bʲ/||P||bP|
|t /t̪ˠ/, /tʲ/||dt /d̪ˠ/, /dʲ/||T||dT|
The other consonants do not change under eclipsis.
2. A vowel receives a preceding n- (pronounced /n̪ˠ/ before a, o, u, /nʲ/ before e, i). The hyphen is not used before a capital letter.
|Radical||Eclipsed||Capitalized radical||Capitalized eclipsed|
|a, á||n-a, n-á||A, Á||nA, nÁ|
|e, é||n-e, n-é||E, É||nE, nÉ|
|i, í||n-i, n-í||I, Í||nI, nÍ|
|o, ó||n-o, n-ó||O, Ó||nO, nÓ|
|u, ú||n-u, n-ú||U, Ú||nU, nÚ|
Environments of eclipsis 
After plural possessive pronouns 
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"
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)
A vowel-initial word is not affected after the definite article an.
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"
A vowel-initial word is not affected after the interrogative particle an.
Changes to vowel-initial words 
Vowel-initial words undergo eclipsis, as explained above, except after an.
A vowel-initial word does not change where 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")
But where neither eclipsis nor lenition is expected, an initial vowel may acquire a prothetic onset consonant t- or h-.
A vowel-initial masculine singular nominative noun takes the prothetic onset t- after the definite article. The hyphen is not used before a capital letter.
- an t-uisce "the water" (masculine singular nominative)
- an tÉireannach "the Irishman" (masculine singular nominative)
The prothetic onset h comes only when both the following conditions are met:
- a proclitic causes neither lenition nor eclipsis of consonants
- a proclitic itself ends in a vowel sound
Examples of h-prothesis
- a haois "her age" (after possessive pronoun a (“her”))
- 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)
- na héin "the birds" (on plural nominative/dative noun after definite article na)
- chomh hard le caisleán "as high as a castle" (after chomh (“as”) [xo])
- 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)