There are nouns in Munster Irish that were spelt -adh or -ódh in the older script that become -aidh and -óidh in the genitive. Some of them do not have an audible slender g, and others do. This suggests, for example, that a word like ampladh (original spelling) has been reinterpreted as ampla, and so the genitive is ampla.

Here is a list of them. There may be more. The following are the correct genitives (note in particular: toraidh, ionaidh and madraidh do not have audible slender g endings, and so the following spellings are advisable in the genitive):

ampla
baiste
barra-thuisle
bíobha
camra
cré (traditionally written créidh)
fógra
iolra
iúnadh
lonnra
madra
masla
peaca
rabhadh? ( I don't think rabhaidh has a slender g, but I lack examples)
scannradh
toradh

These have a slender g in the genitive (some are attested with and without):
adhnóidh
annróidh
bídh
bloscaidh
bua/buaidh
casadh/casaidh
céimaghráidh (the gs of céimaghrá, adapted from the English "chimera")
ceóigh
cogaidh
cómhráidh
curaidh? ("warrior"; I'm not sure here, though, about the slender g)
deiridh
duaidh? (I'm not sure here)
duibh-réidh
gaorthaidh
geímhridh
gleóigh
margaidh
páigh
sáigh (gs of sá, "saw")
samhraidh
slabhra/slabhraidh
sneachtaidh
súigh
troscaidh

One might also venture so far as to call some of these fifth declension nouns (though fifth declension nouns are most often characterised by a broad ending in the genitive), as historically certain words that did not have a final dh take the slender g ending. This happened due to analogy of the other nouns.

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I'm an intermediate speaker of the Corca Dhuibhne dialect of Irish and also have knowledge on the old spelling
Soir gaċ síar, fé ḋeireaḋ thíar

Wouldn't that make them 4th declension?

Fourth declension = nominative is the same as genitive

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I'm an intermediate speaker of the Corca Dhuibhne dialect of Irish and also have knowledge on the old spelling
Soir gaċ síar, fé ḋeireaḋ thíar

At least in traditional grammar

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I'm an intermediate speaker of the Corca Dhuibhne dialect of Irish and also have knowledge on the old spelling
Soir gaċ síar, fé ḋeireaḋ thíar

I've added some more in: sneachta, ceó, gleó, pá and sú.

There is an awkwardness in the spelling. Some are spelt -idh and some spelt -igh above. I'm basically keeping an eye on the spellings already used in Muskerry dialectal literature. And I think monosyllable nouns with a long vowel in the nominative singular like ceó and pá tend to become ceóigh and páigh. Ultimately it doesn't matter - it's more a case of establishing orthographical rules.

Let me add to the list spriocadh, which is identical in the genitive. FGB has sprioctha in the genitive, which is pronounced the same, and so amounts to the same thing.