Firstly, let us preface that these are not exactly English “w” or “v” sounds, but this is how learners typically ask the question in relation to pronunciation. Basically speaking, “w” is a broad bh/mh, and the “v” is a slender one. Consonants that come beside an “i” or an “e” are slender, and the ones that come beside “a”, “o”, “u” are broad.
When learners come across this difference in pronunciation, they are usually looking at words like “bhfuil (i.e. “an bhfuil”) and “mháthair” (i.e. “mo mháthair”). How you pronounce it largely depends on which dialect you choose.
In Munster, both broad and slender bh/mh tend to have a lot of friction, so they both sound like ‘v’, but the broad one has rounded lips, and the slender has flattened lips. Traditional Munster Irish often nasalises stressed vowels next to nasal consonants. In conservative speech, the nasalisation can remain even after the consonant that caused it has weakened or disappeared, particularly where written mh no longer survives as a consonant. Déise Irish developed this especially strongly, often producing nasalised /ãi/ and /ãu/ in environments where no etymological nasal consonant existed.
It’s [w] generally in Donegal, but clusters like “amh”, “abh” aren’t; they usually become a long vowel, e.g. “amhrán” /o:ra:n/, “gabh” /go:/. Also “domhan” /dõ:n/, “samhradh” /sãuru:/. “mh” makes the vowel nasal, but in “amhrán” it doesn’t; it’s an exception (so it’s sometimes spelt “abhrán”). It can be a [v] before r initially, though, too.
In a nutshell: [v] is a voiced labiodental fricative sound, produced between the upper teeth and lower lip, as per the English v. [β] is a voiced bilabial fricative sound, produced between the two lips. Then nasalisation is applied if the consonant is mh rather than bh.
mh (broad and slender)
bh (broad and slender)
Note about Waterford – Déise Irish (Posted with permission from erisceres): The changes in pronunciation can generally depend on where it occurs in the word but also possibly on dialect.
/v/ is pronounced as [w] when:
- in absolute initial position (typically limited to verbs at the start of clauses), e.g. bhuail sé
- preceding /l/ (/vl/ is pronounced [wʁʷ]), e.g. dhá bhulóig (pronounced dhá bhlóig)
/v/ is pronounced as [β] when:
- in intervocalic position when preceded by prefixes and proclitics, e.g. ana-mhaith, mo bhata
in absolute final position, e.g. dubh - preceding other consonants (besides /l/), e.g. dhá bhróig
In most cases, medial bh/mh is vocalised with syllable coalescence, except retained and pronounced [β] when:
- followed by a long vowel, e.g. sábháil
- in most adjectives formed with the -mhar suffix, e.g. ciallmhar (but vocalised in greannmhar)
- in some instances where -mhach occurs, e.g. seasamhach, taitneamhach
- in some isolated exceptions, e.g. siabhra
This can all be essentially summarised as the following:
- if a clause begins with a verb starting with bh/mh (a common enough occurrence), /v/ is pronounced [w]
- if you happen to hear /vl/ occurring (whenever a speaker has the traditional Déise /l/), /v/ is pronounced [w]
- the rest of the time (wherever bh/mh is actually pronounced), /v/ is pronounced [β]
Reference: See Breatnach §§169-173. See also §§501-502 for medial exceptions.