>>> Iss ê benfus glas ndegiunoch for îarcaine Êrenn
> "Is é benfas glas ndéidenach for íarcaini Érenn
> It is he that will obtain the final blue (green?) from the
> woad-bath (green dye bath?) of Ireland
Isatis tinctoria ("glaisin" or "woad") yields indigotin, which
dyes blue, not green. The semantic range of OI "glas" includes
both colors, but we can read it as "blue" here.
See this site for a good treatment of woad dying:
In particular, from the description there I'm led to wonder if
"íarcain" was the Irish term of art for what a traditionalist
modern dyer would call the "indigo vat" or "blue-pot":
> To prepare the dyebath a "reduction" reaction to remove oxygen
> from the solution is required along with a change in the pH
> required to dissolve the dye. The reduction reaction was
> historically done by bacterial action in a vat of indigotin,
> aged urine, wheat bran and other additives that was a long and
> very strong smelling process. This was the traditional indigo
> vat or blue-pot. The fermented urine produced ammonia that
> shifted the pH level of the vat into the alkaline levels required
> for the effective use of the dye.
By the way, for the use of "benaid" ("benfus" above) to mean
"extract, obtain", see DIL B 73.1; FGB "bean 2 = bain 1"; and
of course "1 bain".