In their edition of LU, Best and Bergin report that on page
83b "along the lower marg. in a sixteenth cent. hand is the
following quatrain, stained and obscure at beginning and end:

A deg (?) ruithnecht am bulg dubh.
ni fudhan ni don min gil.
ciasa teimhen mo comainm.
is geal mo toigairm for nim.

I translate this as:

Its fine shine in a dark bag
is of no use to the bright flour;
although my name is obscure
my summons to heaven is bright.

The first half looks like a proverb to me, or the elaboration
of a proverb, although it might also be a newly minted poetic
image.  What do you think?