> > the vowel of the first syllable of "Beltaine" (modern "Bealtaine",
> > Sc.G. "Bealltainn") is historically short
> May-day, Irish béalteine, Early Irish beltene, belltaine
> Béaltaine (Meath) and Beáltaine (Connacht and Donegal)
I'm not too surprised to see long vowels in modern variants.
This is not hard to account for. In many Gaelic dialects a
stressed syllable that consists of a short vowel followed by
ll + hiatus or another consonant regularly -- one might as well
say automatically -- results in the short vowel being either
lengthened or changed to a diphthong. Besides the examples
above, the first syllable in both Scottish Gaelic "Bealltainn"
and in Munster Irish "Bealtaine" are in fact pronounced with
a diphthong, roughly /byaul-/.
What is interesting here is the apparently widespread perception
that the word is properly "Belltaine". The headword in DIL is
in fact "Bel(l)taine", and this spelling with double 'll' goes
back at least as far as Sanas Cormaic, where entry #122 begins
"Belltaine .i. beil-tine .i. tine beil..." LEIA acknowledges
that the word is "souvent écrit avec -ll- double", but does
not address this in the discussion of possible etymologies.
Hey David! Is there some OI phonological process that would
naturally account for a meaningless -lt- / -llt- variation?
Early spellings with a long 'é' (*Bél(l)taine) are lacking,
however, as far as I know. Since only the historical quantity
of the vowel is relevant to the etymology, "bél" (mouth) still
seems to be excluded.