Is there some kind of rule for determining what words take a special vocative form (beyond lenition and, in the case of masculine names, slenderizing the final consonant)? I know that some nouns have a special vocative form (e.g., leanbh/a linbh), but I've never learned if there's a way to know which these are, or if they have to be memorized.

This was inspired by a request at IGTF for "farewell, cousin."

Redwolf

Nouns of the first declension (i.e., the ones who make their genitive singular by slenderising the final consonant in the word) have the same form in the vocative as in the genitive singular. Leanbh, for example, falls into this declension, and the vocative a linbh is completely regular.

In the plural, the subset of nouns in this declension that form the nominative plural the same way as the genitive singular (i.e., by slenderising the final consonant in the word) make form the vocative by adding -a instead of slenderising:

(an) mac / a mhic / X an mhic | (na) mic / a mhaca / X na mac

Nouns of all other declensions are the same in the nominative and the vocative, both in the singular and the plural.

_________________
Not a native speaker.

Always wait for at least three people to agree on a translation, especially if it’s for something permanent.

My translations are usually GU (Ulster Irish), unless CO (Standard Orthography) is requested.

I think there are also some third declension names that add an -a, e.g., Aonghus => a Aonghusa, but I think they are considered more historic curiosities than current forms (unless your name happens to be Aonghus of course.)

Page 106 of Graiméar Gaeilge na mBráithre Críostaí has a few fully declined examples of common masculine and feminine nouns with adjectives. (I bookmarked it. )

_________________
Múinteoir Gaeilge - Irish Teacher
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).

Personal names often don’t quite fit into any particular declension or paradigm, though, so those can be treated as being just irregular forms. Same with Mícheál becoming (a) Mhíchíl in the vocative and genitive, for example.

_________________
Not a native speaker.

Always wait for at least three people to agree on a translation, especially if it’s for something permanent.

My translations are usually GU (Ulster Irish), unless CO (Standard Orthography) is requested.

Micheál becomes either a Mhicheáil or a Mhichíl, depending on the dialect.
In Ulster we only say "a Mhicheáil", as far as I know.

_________________
Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

In Gaoth Dobhair, I’ve only heard a Mhícheáil (though I have, once, heard a Gaoth Dobharian say teach Mhíchíl), but further east, I think there’s more variation. I was in a class with two Michaels from Belfast last year—one of them preferred a Mhícheáil, the other preferred a Mhíchíl.

_________________
Not a native speaker.

Always wait for at least three people to agree on a translation, especially if it’s for something permanent.

My translations are usually GU (Ulster Irish), unless CO (Standard Orthography) is requested.

GRMA! For some reason, this has never been covered in any learning product I've used or in any class I've taken, so I've been navigating blindly.

Redwolf