I've been wondering about this for awhile. There are many ways to spell one Irish word, all correct in my own opinion. For example:

Sgéaluidheacht (my way, I learned to spell Irish here https://knockmealdownactive.com/PDF/An% ... %20Ann.pdf)
Sgéaluigheacht
Sgéuluigheacht
Sgéulaidheacht
Sgéalaidheacht

I could go on for a while changing aidheacht to uidheacht (and with gh insead of dh) and sgéal to sgéul. Also I could spell it with an sc instead of sg.

The only spelling I would call dialectal that I have noticed is West Munster -ibh plural endings (is that ever pronounced like written, by the way?). Buachaillibh, not buachaillí. Are there any other regional rules to spelling or for example my example above, is it purely personal preference whichever province you are from?

-ibh is the dative plural only. Buachaillibh and buachaillibh are dative plural. The nominative plural is still buachaillí. That said, there is (limited) use of the -ibh in other cases, eg "a fhearaibh" for "a fheara" in the vocative plural.

Where used, the -ibh is pronounced like that. "I gcoinnibh na dlí" is pronounce guingiv in the relevant word. I suppose some people say "i gcoinne na dlí" too.

You can also take leathan le leathan, caol le caol to the Nth degree: mío-thráthamhail, but even Dinneen didn't bother with that much.

If you go back far enough, spellings liked to separate broad endings from slender medial vowels, eg "bead", "I will" could be "beidhead". Beifí could be beidhthí etc.

I think Dinneen preferred -dh- in nouns - madraidhe, scéalaidheacht - and -gh- in adjectival comparatives - tuirsighe, I think. That said, there was no consistency in books on this: bliadhain, bliaghain.

éa = eu.
There is no need for an accent in the digraph eu

sgeuluidheacht/sgeulaidheacht

The people I know who use the éu spelling only use it in a few words like béul, sgéul.

It was more common 1910s-1930s.

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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

and it is only appropriate in words where éa is pronounced /ia/, eg béal, sgéal, Béarla, each pronounced /ia/.

In words where éa is pronounced /e:/, eg. téacs, it is not "eu".

Brian Ó Cuív has his own system in CFBB. Whereas Ua Laoghaire wrote sgeul, BÓC writes sgéal, where éa for BÓC denotes /ia/. But in words like téacs where the vowel is /e:/, BÓC writes téux (see CFBB p279). See also éugfaig (CFBB p1) where the vowel is /e:/.

So there were a number of rival implementations.

Also worth noting is that béal etc had /ia/ not /iə/. The second half of the diphthong was very open.

If you listen to https://www.doegen.ie/LA_1034d3, where Amhlaoibh Ó Loingsigh said céad, it was not /k'iəd/, but /k'iad/, like "key+yard" said together very quickly. I think most speakers have /iə/ nowadays, and not /ia/, and so this point of pronunciation has changed. It's actually quite difficult to say Béarla with a very open second element in the diphthong. I this that is what linguists call a rising diphthong.