In the meantime....
In the 1887 book "HANDBOOK OF THE ORIGIN OF PLACE-NAMES IN WALES AND MONMOUTHSHIRE by The Reverend THOMAS MORGAN, DOWLAIS, MERTHYR TYDFIL."
we have the following:
Ach, a stem, a pedigree, a river; ddu, inflection of du,
black. We find ach in Clydach, Mawddach, &c.
Barmouth. — An Anglicized form of the Welsh name, Abermaw, which signifies a place situated at the mouth
of the river Maw. Bar is a modification of Aber, and mouth of Maw, or Mawddwy. Maw means broad,
expanding. Mawddach, according to Dr. Owen Pughe, means overflowing water. " Oedd maw ei rhydau,"
broad were its fords. The Anglicized name was adopted in 1768 by the seafaring fraternity in order to have
an English name inscribed upon the vessels. English name — Broadmouth.