Zurni wrote:
An Cionnfhaolach wrote:
A few words change drastically in the vocative:
Micheál becomes a Mhichíl and my own name Cian becomes a Chéin.
Cian
So...that is only learning....no one can explain why in this cases?
a Mhíchíl instead of a Mhícheáil is really an exception.
Cian - a Chéin is a usual change of -ia- to -éi-. (though it does not occur in every case of -ia-: Brian - a Bhriain, not a Bhréin.
Compare with iasc - éisc (fish, of fish/fishes)
Nouns differ according to their declension group. There are (at least) 5 groups.
The only declension group with a change in vocative is the 1st declension.
In first declension vocative is the same as genitive: fear - a fhir, mac - a mhic, amadán - a amadáin, iasc - a éisc (in case you talk to a fish)
All nouns here are masculine.
In every other declension group (and most irregular nouns) vocative is the same as nominative: múinteoir - a mhúinteoir, bean - a bhean, iníon - a iníon, etc.
That's why all feminine nouns and all female personal names don't change in vocative. (None of them can be in first declension)
Unfortunately

most male personal names belong to first declension.
So, there are only a few male names without change in vocative, e.g.:
Liam, fourth declension (nominative = genitive = vocative, no change at all): Liam, teach Liam (Liam’s house), a Liam (Liam!)
Aonghus, third declension (genitive: Aonghusa, vocative Aonghus): Aonghus, teach Aonghusa, a Aonghus. (though sometimes it is used as a first declension noun: a Aonghuis)
Críostóir, third declension (genitive: Críostóra, vocative Críostóir): Críostóir, teach Chríostóra, a Chríostóir.
Mac is first declension, Ó is irregular with a vocative (and genitive) in Uí
So, most Irish surnames must change in vocative, too.