I'm in the middle of a 40-odd page article about lenition in Muskerry Irish, but it will take some time to complete. Maybe I'll post some more questions over the weekend on awkward areas of lenition, of which there are many!!

A Lon Dubh: there are so many fiddly issues with numbers.
With ar+numeral, we have lenition: ar thrí (ar thrí cinn de chaoribh) and so:

ar cheithre X
ar chúig X
ar sheacht X
ar ocht X
ar naoi X
ar dheich X

The odd one out is: ar sé X.
Admittedly there are few relevant examples, but in 3 Kings 10:29 (Peadar Ua Laoghaire's translation) we read Agus tháinig carbat cheithre gcapall as an Éigipt ar sé chéad sicil airgid.

Do you have an explanation for this?
Maybe "ar shé" would sound quite like "uirthi"? Or maybe "ar se" (ar+6) is influenced by the phrase "ar sé" (said he)? [Although the latter is normally "ar seisean".]

You will sometimes see or hear "ar seacht". An example from Croí Cainte Ciarraí:
ar seacht scillinge ist ló

Similarly Maidhc Dainín Ó Sé's "Lili Frainc":
ar seacht neomant

So I think the case of "ar sé" and "ar seacht" is probably a case of dentals sandhi with lenition being blocked. I think "sé" is more likely to resist lenition than "seacht" for the reason you mention and also there is a tendency that the dentals rule is more likely to be kept to if a long vowel follows.

Also in terms of dentals, Amhlaoibh Ó Luínse has "ar deich" in Scéalaíocht Amhlaoibh Uí Luínse in "Muracha agus Maonas".

Ar deich is more common than ar dheich in Croí Cainte Ciarraí.

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The dialect I use is Cork Irish.
Ar sgáth a chéile a mhairid na daoine, lag agus láidir, uasal is íseal

Thank you, that's interesting.
And in Seanachas Amhlaoibh p56, there is: Tugaim tínteán ar trí nú ceathair do throithe amach ón iarta. [Here the fact that "ar" governs a whole phrase may play a part.]

In Lughaidh mac Con by Ua Laoghaire, I've just found (p36): Curtar ar seacht miasaibh fichid iad anois [Ua Laoghaire frequently wrote fichid for fichead, but this is not the point of citing this here.]

Are these right?
Her two apples: a dhá húll.
Their two apples: a dhá n-úll

I can't find direct attestation of h and n use after a dhá.

From the book "Sceulta Mhicil Uí Mhuirgheasa ón Rinn" on p.21:

"a dhá h-uillin leagtha ar a bharra"

From Jimín Máire Thaidhg, p.34 of the 1st Edition:

"ceanglaimís a dhá n-eirball!"

Maidhc Dainín never seems to have it. I'll have a look through more of my folklore collections to see if I can find other examples.

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The dialect I use is Cork Irish.
Ar sgáth a chéile a mhairid na daoine, lag agus láidir, uasal is íseal

Still pouring through texts, but note the following from Scéalta agus Seanchas Phádraig Uí Ghrífín p.61:

D'imíodar amach sa choill agus cailleadh ann iad agus d'imíodar ina dhá cholúr ghléigeal an spéir in airde mar fuaireadar bás naofa

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The dialect I use is Cork Irish.
Ar sgáth a chéile a mhairid na daoine, lag agus láidir, uasal is íseal

That's interesting, particularly as it is ghléigeal and not ghléigeala.

Out of interest, here's an example of possessive + dhá following mutation of the possessive from Cois Fharraige. Seanchas Jimmí Chearra Chois Fharraige, p.21:

Bhí chuile dhuine dhá raibh istigh ar a dhá nglúin...

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The dialect I use is Cork Irish.
Ar sgáth a chéile a mhairid na daoine, lag agus láidir, uasal is íseal

Yes - and interesting there is any numeral there at all, as the English would be "on their knees".