On Fri, 9 Jan 1998, Richard Coates wrote:
> Would anyone care to suggest an authoritative absolute date-range
> for (a) the loss of sibilance, (b) the complete loss of Goidelic
> intervocalic and final /s/? Specifically, how late is it likely that
> there was any phonetic sibilance at all in reflexes of the _s_-stem
> genitive singular desinence _-esos_?
McCone (in _Stair na Gaeilge_, 1994) gives the following progression
for that ending, using OIr "nem" (= neamh, heaven) as an example:
Common Celt. Insular Primitive Irish OIr
*nem-es-os -> *nemisos -> *nev~iyah -> *niv~eya -> nime
McCone places the loss of final -h logically and chronologically
between the "Irish lenition" (which turned the voiceless stops t,k
between vowels into voiceless fricatives) and "general apocope"
(which deleted final short vowels). He places the first event
sometime between the beginning of Proto-Irish and the middle of the
Primitive Irish period, and he elsewhere defines Primitive Irish as
existing during the 5th and 6th centuries AD, and Proto-Irish as the
language that existed up to the end of the 4th century. He dates
general apocope to around 500 AD. Once you sort all this out, you
end up with a rather generous swath of time extending from sometime
before 400 AD up to 500 AD.
McCone's arguments are available in English in the more recent
_Towards a Relative Chronology of Ancient and Medieval Sound Change_