When Gwynllyw was dying he was attended both by his son Cadoc and by Saint Dubricius, who administered the last sacrament to him. The traditional date of his death, 29 March, is the day dedicated to him. The year of his death is uncertain; suggestions include 500 and 523. Following his death his hermit cell became an important shrine and a church was built there. This is now St Woolos Cathedral, the seat of the Bishop of Monmouth. In the 9th century Gwynllyw's church was rebuilt in stone, indicating his importance and the wealth of his shrine, as stone buildings were unusual in Wales at that time. Part of this building is now incorporated into St Woolos Cathedral as the Galilee chapel.