Is it the description the issue ? Rhos Fynach was never a "monastery", it was an outlying farm of the Abbey of Aberconwy. If the question relates to Caernaronshire, then that could be a simple error by a writer on a 20th century postcard. Llewelyn the Great granted to the Abbey of Aberconwy the "grange of Creuddyn" which included Rhos Fynach.