Analysis of information sources in references of the Wikipedia article "Dolwyddelan" in English language version.
These were men inhabiting this place before the grandfather of Sir John [Wynne] migrated here from Merioneth. The castle was built in the fifth century — very nearly a thousand years before his grandfather moved here, and two hundred years before Saint Gwyddelan was born.
I do not want you to spell the name of my birthplace with the w. The name of the place is not derived from St. Gwyddelan... There are many old roads called "Ffyrdd Elen" and a well called "Ffynnon Elen" here, but no place called after St. Gwyddelan. It is evident this place was named Dolyddelen to commemorate Elen Llueddog