This story concerns the most ancient Irish Celtic gods, the first generation of the Tuatha dé Danaan who had to fight off the giant races of the Firbolgs and the Formorians. Their history is found in the Lebor Gabála, The Book of Invasions.
When the fairy race of the Tuatha dé Danann arrived in Ireland, they came like a mist across the waters, bringing with them magical gifts. These were the lia fail the coronation stone, the spear of Lugh, the sword of Nuada, and the great cauldron of the Dagda, which was said to be able to restore life.
The Dagda himself was known as the Good God and he was chief of the gods at this time. Besides his cauldron, he had a harp which was battle-scarred and made of oak. It was covered in rich decorations including a double-headed fish which ran up and down the curved pillar and had jewels for its eyes. Although he had a harper, Uaithne, he could also play it himself.
The Dagda had this harp with him always he even took it into battle. So it was, that after the second Battle of Mag Tuiread, or Moytura, the Dagda discovered that his harp, together with his harper, had been captured by the Formorians and taken with them in their flight. Angered beyond measure, he set out with his son Aengus Og to reclaim it.
Stealthily they approached the Formorian camp. Soon they could hear the sounds of the feasting hall in which Bres, the Formorian king, was dining. Approaching the doorway, they could just make out through the smoke and candle-flame the outline of the old harp hanging on the wall. Then the Dagda entered boldly and summoned his harp with this chant:
"Come Daurdabla, apple-sweet murmurer
Come, Coir-cethair-chuir, four-angled frame of harmony,
Come summer, come winter,
Out of the mouths of harps and bags and pipes!"
Immediately the old harp flew to his hand across the hall, killing nine men as it came. A shocked hush fell on the company. In the silence the Dagda laid his hands on the strings and unleashed the Three Noble Strains of Ireland that he had bound into his harp. First he played the goltrai, or strain of weeping, so that all present began to mourn and lament their defeat. Then he played the geantrai, the strain of merriment, so that the company turned to laughter and drunken foolery. Lastly he played the suantrai, or sleep-strain, whereupon the warriors fell into a profound slumber. After this the Dagda and Aengus Og left the camp as quietly as they had come, taking Uaithne and the harp with them.