This tale about Eithne belonged to the Mythological Cycle. The title, Altrom Tig Da Medar or "The Nurture of the Houses of the Two Milk Vessels" is preserved in the manuscript known as the Book of Fermoy.

The story actually begins after the Milesians defeated the Tuatha De Danann, where the three sons of Cermait (grandsons of Dagda) were killed in the battle of Tailtiu (see the Coming of the Milesians). Manannán MacLir helped the surviving Danann to retreat from the sights of mortal men to one of the many hidden domains, known as sidh.

Manannán gave feth fiada to the Tuatha De Danann, the power to remain invisible from mortal eyes. The sidh is hidden with magical shrouds, and only a person with feth fiada can come and go from the sidh. Without the feth fiada, a person could not possibly find his way back to his sidh.

Manannán also gave each sidh the Fled Goibniu (Feast of Goibniu) and the Pigs of Manannán. The former are drinks, possibly ales, which keeps the Tuatha De Danann young. While the later can be slaughtered, cooked and eaten; but in the morning of the next day, pigs are alive and to be consumed again at the feast.

There are many sidhs around Ireland, and each sidh is ruled by Danann king or queen or both. The sidh was magical place, sometimes known as a fairy fort, but it usually contained a palace. At Sith Buidb on Lake Derggert, the sidh was ruled by Bodb Derg, while Midir ruled in Sith Truim. But the most famous otherworldly kingdom in Ireland was the Brug na Boinne, the sidh of Elcmar. Elcmar was husband of Boand or Boann and the foster father of Angus Og. Angus' mother is usually Boann, whom Dagda secretly seduced.

Not long after preparing the new homes for the Tuatha De Danann, Elcmar invited everyone (Danann) to a feast that he has prepared. Manannán advised Angus Og that he should expel his foster father from the Brug; since it was prophecised that Angus would take it. After teaching Angus a powerful spell, Elcmar was forced to abandon his realm, so Angus became the new lord of Brug na Boinne.

Here, Manannán told Angus the secret that there was one God above all others. Manannán disclosed about the creation of the world and the first man, Adam.

Like all Celtic family, Angus Og decided to foster many children of the Tuatha de Danann. Among the fosterling in his care was Curcog, daughter of Manannán. Around this time, Angus' steward, Dicu, had a wife who was pregnant. She gave birth to a girl named Eithne. She too was brought up in Angus' court and became a dear companion of Curcog.

Eithne grew into a beautiful young woman and wooed by many suitors, but problems arose when one suitor named Finnbarr, insulted and upset her. After this insult, she would not eat any food or drink at Angus' table. After seven days of fasting, Angus became seriously concerned. Neither her foster father (Angus) nor his druids knew why she refused all food and drink. When Eithne would only drink from what Angus gave her, in a golden goblet filled with milk of his Dun Cow. After this, she ate and drank nothing else. Eithne herself would milk the cow into the golden goblet.

Years and centuries passed and still Eithne's only diet was the milk from her foster father's Dun cow. Hearing of this strange ailment, Manannán sent for her to his kingdom at Emain Ablach. Manannán also tried to make a meal for Eithne, but she wouldn't touch these food and drink. When Manannán gave her a drink of milk from a golden goblet, the Danann maiden accepted. The milk comes from Manannán's own Speckled Cow.

Manannán then realised that the causes of Eithne's diet. When Finnbarr insulted her, the demon within her body left her, replaced by an angel. This angel would not accept any food of the Tuatha Da Danann, but would accept milk from the dun and speckled cows, because it came from the land of the righteous - India. Eithne was no longer fully Tuatha De Danann.

Eithne returned home to Brug na Boinne, where she continued to live for many centuries, drinking only milk from the Dun Cow.

One day, when it was especially hot, all the girls in Angus' household went to bathe in the river. At this point, all her friends had disappeared. She could not find Curcog or the other ladies. She had lost her power of feth fiada that she couldn't find her way back home.

In her search for either her friends or her home, she found way to the church, where St Patrick greeted her. Here, she learned about Jesus and Christianity. She stayed with Patrick for a number of years.

Angus Og and Curcog were upset with Eithne's disappearance. They search for her everywhere, until they found Eithne in the church. Both foster father and priest argued where Eithne belonged to. Though, Eithne loved and missed her home, her foster father and friends, she chose to stay with the cleric and accept the new faith. Angus and the girl's friends grieved for her when she told them of her decision.

Eithne immediately confessed her sins and was baptised, because she felt that her death was near. She died a couple of weeks later and was given proper burial. And we are told that her soul went straight to heaven.

There is very little myth about Lir, the god of the sea. Even in the story about his children in Oidheadh Chlainne Lir (Death of the Children of Lir), Lir had very little role in the tale, beyond marrying two sisters and fathering four children. This tale is one of three in the Three Sorrows of Storytelling, written in the 16th century.

Lir had four children by his second wife Aeb. Their names were Fionuala, a girl, and three boys – Aed, Fiachra and Conn. When Aeb died, Lir married Aeb's sister Aiofe. Aiofe, who was childless, became jealous of Lir's love for his children.

One day, pretending to take the stepchildren to visit her foster father Bodb Derg, Aiofe transformed the children into beautiful swans. Aiofe also placed a terrible geis on her stepchildren, so they would wander through Ireland and Britain for nine hundred years, before they could restore of their original human forms.

When Bodb Derg discovered Aiofe's action against her stepchildren, Bodb turned her into a demon.

(Please note that there was another Aiofe, who was the wife of Manannán, who was transformed into a crane. I am uncertain if both Aiofe are one and the same person.)

For nine hundred years, the swan-children wandered throughout Ireland and Britain, suffering from hardship, but they became famous because of their beautiful singing. Even they were transformed into swans; the children of Lir retained their human voices. The Danann would often come to them and listen to their songs.

They finally found refuge from a hermit, named Mo Cháemóc, before they were transformed back to their human forms. The children of Lir were old and writhed; they were dying from old age. The hermit immediately baptised them before their death and had the children of Lir buried together in a single grave.

The Oidheadh Chlainne Lir has one of the typical themes found in myths and folklore - that of the wicked stepmother. The magical transformation of a person into an animal is also typical in fairy tale, except here, there are no happy ending, unless the moral of story of changing converting one from paganism to Christianity.