Dennis wrote:

> > I think I read somewhere (maybe in GOI) that single "s" is more
> > consistently used after long vowels, e.g. "grés". But I can't say if
> > the evidence really bears this out.
> My gut feeling is that 'ss' could be used like initial 'h', to add
> bulk or "presence" to a small word.  A word like "grés" is visually
> "small", but the long vowel gives it more stature than "ass", to the
> ear at any rate.

Thurneysen isn't very concrete about "s" vs. "ss". In a chapter on geminates he writes:

§ 144. They [i.e. geminates; by definition almost all internal "s"s of OIr. are geminates etymologically] are most frequently written double between stressed short vowel and
another vowel, and also in final position after a stressed short vowel,...; in later sources ss appears less consistently...

He cites forms like fiuss, fiss (Wb) and fius fis (Sg.), nessa, nesa.

§ 145. After stressed long vowels geminates are more commonly written single; e.g. ... césad "suffering" oftener than céssad; ... ·rísa for ·rís-sa "I may come".

§ 146. Geminates are also simplified after all unstressed vowels, especially in final position... follus "clear" (foluss Sg. 40b14); is "is", seldom iss; isolated tairisem
"standing fast" beside usual tairissem...
So too, where a pretonic word is run on to a stressed word: isamlid often for is samlid "it is thus"...

In another chapter he cites a few interesting spellings:

§136. ...Normally scribes refrain from doubling both consonants in an unlenited group, as in cosscc "correction" Wb. 9a23; instead they, they geminate now the one or the
Examples: béssti "beasts" Wb. 31b21; dussceulat (du·scéulat) "they experience" Ml. 83b8...