I want to make some quick notes on the vocative particle here after seeing some inaccurate information on the Internet about it, particularly on the BiteSize Irish Gaelic website.
The particle is a, which lenites the following word.
* If the name begins with a vowel, the pronunciation is zero. A Eóin! is just /oːnʹ/. Nothing of the vocative particle is heard, as it is fully elided in the following vowel. BiteSize Irish says the following:
Áine (AWN-yeh) becomes a Áine (uh AWN-yeh)
Eoin (OH-in) becomes a Eoin (uh OH-in)
Aodán (AY-dahn) becomes a Aodáin (uh AY-dah-in)
These are unfortunately all incorrect. A Áine is just /ɑːnʹi/; a Eóin is just /oːnʹ/; and a Aodáin is just /eː’dɑːnʹ/.
* If the name begins with a broad consonant, the pronunciation is a neutral vowel, /ə/. BiteSize Irish correctly states:
Máire (MOY-ruh) becomes a Mháire (uh WOY-ruh)
Sorcha (SUR-uh-khuh) becomes a Shorcha (uh HUR-uh-khuh)
However, the vocative particle is brief to non-existent, and the lenition suffices to show the existence of the particle. A Mháire is /ə vɑːrʹi/ or just /vɑːrʹi/ in fluent speech. A Shorcha is /ə horəxə/ or just /horəxə/ in fluent speech.
* If the name begins with a slender consonant, the pronunciation is an i-coloured neutral vowel, /i/. BiteSize Irish states:
Seán (shawn) becomes a Sheáin (uh HYAN)
Séamas (SHAY-muss) becomes a Shéamais (uh HAY-mish)
But to be more accurate, a Sheáin is /i xʹaːnʹ/ and a Bhriain is /i vrʹiənʹ/, and here too the vocative particle can be ‘overdone’: in fluent speech the particle would often be barely audible. These would be more likely to be heard as /xʹaːnʹ/ and /vrʹiənʹ/. A Shéamais is a special case, because it seems slender s is lenited to /xʹ/ in some cases, especially before back vowels, and to /h/ in other cases, and /h/ doesn’t have a broad-slender constrast, and the transcription here should maybe be /ə hiamiʃ/, with /hiamiʃ/ being the realisation.
The important thing is the lenition, not the particle itself, which is often only theoretically there. Otherwise learners of Irish will be speaking stilted Irish, just as stilted as learners of English who were taught to say thee cat sat on thee mat, with the word the pronounced as thee wherever it is found.