Diwrnach was born in Ireland, a land that the Welsh called Iwerddon, and he served as steward - or maer, a term that meant both purveyor and chamberlain - to King Odgar son of Aedd, the ruler whose name appears in the Welsh genealogies as a king of sufficient standing that Arthur himself would treat with him as an equal. The name Diwrnach derives from the Irish Diugurnach, a rare but attested name, and the Welsh form Wyddel that follows it means simply "the Irishman," a label that marked him as foreign in the world of Culhwch and Olwen. But he was not merely a servant. He was a giant, a man of such stature and pride that he kept the most famous cauldron in Ireland as his personal possession, and he was wealthy enough to maintain a retinue of warriors and a household that could entertain a king.

The defining crisis of Diwrnach's life began not in Ireland but in Wales, where Ysbaddaden Chief Giant had set Culhwch forty impossible tasks as the price for marrying his daughter Olwen. Among those tasks was the procurement of the cauldron of Diwrnach the Irishman, a vessel needed to boil the meat for the wedding feast. King Arthur, Culhwch's cousin, sent an embassy to Odgar requesting the cauldron, and Odgar commanded his steward to hand it over. But Diwrnach refused. The text of Culhwch and Olwen records his words with blunt pride: "Heaven is my witness, if it would avail him anything even to look at it, he should not do so." The embassy returned to Britain with this denial, and Arthur resolved to take what had not been given.

Arthur's expedition to Ireland was one of the first foreign wars in Arthurian literature, and it was conducted with the swift brutality that would characterise all of Arthur's campaigns. He took a small retinue and sailed in Prydwen, his ship, to the harbour near Diwrnach's house. The hosts of Odgar saw their strength, and Diwrnach received them at his table, where they ate and drank as much as they desired. When Arthur demanded the cauldron for a second time, Diwrnach answered that if he had intended to give it to anyone, he would have given it at the word of his own king, but since he had not given it at Odgar's word, he would certainly not give it at Arthur's. The refusal was absolute. At that moment, Bedwyr, Arthur's champion, seized the cauldron and placed it upon the back of Hygwyd, Arthur's servant whose office was always to carry the king's cauldron and place fire under it. The theft was open, and Diwrnach's response was immediate.

The death of Diwrnach was as swift as it was total. Llenlleawg the Irishman, one of Arthur's warriors, seized Caledfwlch - Arthur's own sword, the blade that would later be known as Excalibur - and swung it in a circle. In a single sweep, the sword killed Diwrnach and his entire retinue. The hosts of Ireland came to fight them, but Arthur and his men drove them off completely, boarded Prydwen with the cauldron now full of Irish treasure, and sailed back to Britain. They disembarked at Porth Cerddin in Dyfed, and there the cauldron was measured and found sufficient for the task it had been stolen to perform. The violence of the episode is striking even in the context of Culhwch and Olwen, a tale filled with giant-killing and monster-slaying: Diwrnach was not an enemy of Britain, not a traitor, not a monster. He was simply a man who owned something that Arthur needed, and who refused to part with it.

The legacy of Diwrnach extends far beyond his death in his own house. His cauldron was later listed among the Thirteen Treasures of the Island of Britain, where it appears as the Cauldron of Dyrnwch the Giant, a vessel that discriminated between cowards and brave men by boiling only for the latter. The name Dyrnwch, meaning "Fist" in Welsh, is thought to be a Welsh approximation of the foreign name Diwrnach, and the giant who owned the cauldron in the later tradition is almost certainly the same Irish steward who died in the earlier tale. Scholars have drawn connections between Diwrnach's cauldron and the cauldron of the Dagda, the Good God of Irish mythology whose vessel of plenty never let any company go away hungry, and between both cauldrons and the Holy Grail itself, the sacred vessel that would sustain the pure and deny the unworthy. In the poem Preiddeu Annwn, the Spoils of Annwfn, Arthur journeys to the Welsh Otherworld to seize a cauldron with the same property - it does not boil meat for cowards - and many scholars believe that the Irish Diwrnach episode in Culhwch and Olwen represents an attempt to euhemerise an older Otherworld tale, replacing the king of Annwn with an Irish king and his steward. Whether Diwrnach was a historical Irishman whose name was borrowed for a myth, or a mythic figure whose story was given an Irish setting, his cauldron and his death remain among the oldest artefacts of Arthurian legend.