A Irish giant. He was steward of Odgar of Ireland. Diwrnach owned a magic cauldron, which would only boil if the meat placed in it was intended for a brave man. It was listed in Welsh lore among the Thirteen Treasures of the Island of Britain.

As one of his tasks, Culhwch had to obtain this cauldron from Diwrnach for boiling meat at Olwenís wedding feast. When Arthur, on the behalf of Culhwch led an expedition to Ireland, requested the cauldron, Diwrnach refused. At this refusal, Arthurís warrior Llenlleawg killed Diwrnach and took the cauldron, which Arthurís warriors filled with Irelandís treasure before returning to Britain.

The appearance of a wondrous Irish cauldron in this tale leads to a possible identification between Diwrnach and the Daghdha and later with the Holy Grail.