I notice that Muskerry or PUL at any rate has a loving of plurals in -sí:

cuntas, cuntaisí
dualgas, dualgaisí
pionós, pionóisí
rialtas, rialtaisí

these are all attested in PUL's works.

I note that PUL has treabhchasaí for tribes, where treabhchaisí would be more logical - and also the form used in Seanchas Amhlaoibh Í Luínse.

PUL also has maitheasaí. I don't think I have seen a slender s version of that. I haven't found any attestation of tinneasaí yet, but I presume this is broad s too.

I am wondering why it is cuntaisí, dualgaisí, pionóisí and rialtaisí, but maitheasaí with a broad s. The only thing I can come up with is that the genitive is maitheasa, so there is never a slender s in this word. Whereas all the others have a slender s in the genitive singular.

This raises the interesting question of why PUL has treabhchasaí and Amhlaoibh has treabhchaisí. did one have treabhchasa and the the other treabhchais in the genitive? Attestation is awaited with great interest.

Similarly for teinneas, where the gs. is teinnis, that would imply a plural teinnisí and not teinneasaí, but maybe it is rarely used in the plural so real examples are hard to find...

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I notice that Muskerry or PUL at any rate has a loving of plurals in -sí:

That's the norm in Corca Dhuibhne also and probably across most of Munster.

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I am wondering why it is cuntaisí, dualgaisí, pionóisí and rialtaisí, but maitheasaí with a broad s. The only thing I can come up with is that the genitive is maitheasa, so there is never a slender s in this word. Whereas all the others have a slender s in the genitive singular.

I think it depends on the word. Index 1 in Dáithí Ó Luineacháin's Cnuasach Focal ón gCom gives a long list of examples where the plural form differs from that given by Ó Dónaill. Here are the words ending in -as or -is in the nominative singular which I have taken from Ó Luineacháin. I give the nominative singular first, then the nominative plural:

aidhmeannas - aidhmeannaistí
aiteas - aiteasaí
amhras - amhraistí
bainis - bainisí
barántas - barántaisí
bronntanas - bronntanaisí
buaiceas - buaiceasaí
carghas - carghaiseanna
ceartaos - ceartaoistí /e:/
cleamhnas - cleamhnaistí
coicíos - coicístí
comórtas - comórtaisí
cuas - cuaiseanna
cuntas - cuntaisí
déantús - déantúsaí
deoiseas - deoiseasaí
dualgas - dualgaisí
eolas - eolaistí
fáltas - fáltaisí
fás - fáiseanna
feisteas - feistisí
fiabhras - fiabhraistí
gabháltas - gabháltaisí
tinneas - tinneasaí (as you were wondering above!)
gléas - gléiseanna
iarratas - iarrataisí
iontas - iontaisí
lamháltas - lamháltaisí
long - loingeasaí (the plural here, though, is clearly from loingeas!)
meaingeal - meaingealsaí (obviously -s plural from English + Irish -aí)
oideas - oideasaí
oireachtas - oireachtaisí
pálás - páláistí
parlús - parlúistí
praghas - praghaiseanna
próiseas - próiseasaí
ráiteas - ráiteasaí
riachtanas - riachtanaisí
sadhfras - sadhfraistí
saghas - saighseanna
sócamas - sócamaistí
spás - spáiseanna
spladhas - spladhaiseanna
tabharthas - tabhartaistí
teastas - teastaisí
turas - turaiseanna
údarás - údaráisí

As you can see -stí is by far the most form plural ending. I think what we're seeing in such forms is a double plural. Take "tabhartaistí" in which the slenderising of the final consonant as is the norm for a first declension noun in the nominative plural and then a t and then the plural suffix -í tagged onto the end. Note that not all nouns have the t - "riachtanaisí" for example. Such double pluralisation is not unique to words ending in -as in the nominative singular, there are other cases: baile - bailte - bailtí - bailtíocha, for example. There are many other examples in Conamara Irish.

The -s plural is also an interesting point to note. Examples from CD are:

scoláirthíos
gugaíos
búidíos
cíníos (breasts)

And this example given to me by a colleague:

Luan na mBan mBeadaíos - the Monday before Ash Wednesday

Well, you don't list rialtaisí there - but I have it attested in Niamh.

Some of these are either/or: cleamhnaisí and cleamhnaistí are both found in Séadna.

I am wondering why it is tabharthaistí but bronntanaisí? There is no reason - you just have to learn them. Unless it is the more common words that would go in -stí, and the less common in -sí? That would give some logic to it.

Now I need to get that cnuasach focal - I didn't know it was Pádraig Ua Maoileoin who put it together!

I don't believe that it has anything to do with the more common words vs the rarer ones. As you say, some words may alternate between -sí and -stí. -saí is quite rare, interestingly enough.

Ó Sé (section 180) gives the following which have a -tí ending in the plural: áras, compás, fearas, treabhchas. He also give déantúistí while Ó Luineacháin gives déantúsaí so as I said forms may alternate depending on the speaker. As I said above, baile can have two plurals: bailte and bailtíocha.

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Now I need to get that cnuasach focal - I didn't know it was Pádraig Ua Maoileoin who put it together!

Yes. The great Pádraig Ua Maoileoin - or Peaidí Ma'leoin, as he's more commonly known - collected the words. A colleague of mine knew Peaidí very well and often speaks of him. I don't know if there's anyone left nowadays who would have the amount of Irish that he had.

Ach dh'imigh sin agus tháinig Monnakawn . . .