>"Bachuil" is not only the name of the crosier, but also the estate of
the >Keeper, and the Keeper himself."
Similarly, the person who carries the formal 'black rod' into our
parliament is himself addressed as 'Black Rod'.
>"Arwachyll" is quite obviously "ar" + "bhachaill", which strickly
>interpreted would mean "battle of bachuil".
A further comment: as it is a compound noun, and assuming for the sake
of argument that Bachaill is genitive, it wouldn't mean "battle of
bachaill" but "of battle-bachall". Which doesn't sound very likely.