On the green pasture dwelt Glas Gaibhnenn, the cow with green spots, in the low country near the spring of sweet water where the grass grew tall and lush. She was of the lineage of Auðumbla and Kamadhenu, and Hathur was her cousin; her milk flowed bounteous and thick as the best cream, and whenever she was studded she gave birth to twin calves.
Now Glas Gaibhnenn belonged to Goibniu the Smith, in the Ridge of Fire, where he lived with his brothers. One day there came to Goibniu the giant Balor of the Fomorians, Balor of the Evil Eye, and Balor hired Goibniu to help build him a castle in his own land across the straight, on a rocky islet known as Tor More, for which he would need nails and hinges, chains and joints of iron, and frames of lead for the windows. So Goibniu left Glas Gaibhnenn in her pasture, under the care of his brothers, who would go out to milk her in the morning, and see her returned safely home at night.
Now Balor lived under a doom, that he would only be slain by his own grandson. Wily was Balor, and locked up his only daughter Ethnea in the top floor of his tower, which formed the donjon of the keep that Goibniu was to build, and never let her out or to see another soul. Yet Balor was wary and wise, and so he ordered his castle built, as another layer of defenses about his precious daughter, whom he could not bring to kill but dare not let marry for fear of the doom promised to him.
Now Cian was brother of Goibniu, and so too was Samthainn. It was to Cian that Goibniu entrusted his cow, Glas Gaibhnenn of the green spots. But Balor knew of Glas Gaibhnenn, and lusted after the precious cow, who could swell his own herd. So with Goibniu gone, Balor rowed across the straight, and commissioned from Cian a great sword. So Cian set to work, and left Samthainn to watch Glas Gaibhnenn. Yet as Balor was set to leave, he went not straight to his boat, but wandered over to the pasture where Samthainn had watch over the cow with green spots. There the Evil Eye shared with him a flask of mead, and with careful half-truths intimated that Cian and Goibniu had treated Samthainn poorly, in leaving him to watch the cow, and were besides using up Samthainn's third of the steel and iron.
Samthainn, enraged by drink and lies, flew back to the brothers' forge, where Cian labored on Balor's sword. While they squabbled, Balor loaded peaceful and wise Glas Gaibhnenn onto his boat, and had already rowed halfway across the strait by the time that Cian and Samthainn were done with their argument. Both brothers berated themselves, but neither so much as Cian, who felt it was his fault, for Goibniu had left the cow with green spots in his care. Yet more, he feared Balor of the Evil Eye.
Now Biróg of the Mountain was a druid, and one of the first of the Tuatha Dé Danaan to come to those shores. She had foreseen the wars of the Tuatha Dé, and had sought out and learned the doom of Balor. So as Balor set out across the strait with his prize, Biróg came up to the side of Cian and spoke to him of her plan. Under Biróg's guidance, Cian shaved his beard and plaited his hair, and hid himself in women's clothes, and so the two sailed across the strait, where Goibniu yet worked as a smith on Balor's castle. To Balor, Goibniu introduced Cian and Biróg as his kinswoman, a widower and her daughter, in need of work, and Balor took them into his household to wash and clean, cook and spin, and care for his imprisoned daughter Ethnea.
Now in time Cian and Ethnea grew close and fell in love, and he would visit her in the night and in rare moments when none else of the household looked, and as the castle rose around her tower, Ethnea in secret gave birth to three sons. Long did Cian, Ethnea, and Biróg labor to keep the children secret, yet once did one of them cry out, and Balor was roused to fury and stormed his daughter's rooms, discovering the great deception. Wroth he was, and cast the three grandchildren into the sea ere his doom could fall upon him...yet one, Lugh, survived.
Now in later years would the revenge of Lugh come on his grandfather, and the doom of Balor would come as had been foretold, though neither Cian or Biróg would live to see it. But that is another tale, and one of greater interest to most, for it deals with kings and wars, and in such great tales the tellers have quite forgotten about Glas Gaibhnenn, the cow with the green spots.