Boann

"She of the white cattle".
Irish goddess goddess of bounty and fertility, whose totem is the sacred white cow. Also goddess of the River Boyne.
She is the wife of the water god Nechtan or of Elcmar, and consort of the Dagda, by whom she was the mother of the god Aengus.
To hide their union from Nechtan, Boann and the Dagda caused the sun to stand still for nine months, so that Aengus was conceived and born on the same day.
FROM: Boann (pantheon.org)
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There are a couple of variations on the myth of the goddess Boann, but one element tends to remain the same - she was the wife of Nechtan, a god of the water. Likewise, Boann was herself a water-goddess, and one of her myths concerns the water. According to legend, there was a sacred well (Sidhe Nechtan) that contained the source of knowledge. All were forbidden to approach this well, with the exception of the god Nechtan (as was noted, Boann's husband) and his servants. Boann ignored the warnings, and strode up to the sacred well, thus violating the sanctity of the area. For this act, she was punished, and the waters of the defiled well swelled and were transformed into a raging river, a river that pursued her. In some versions, she was drowned; while in others, she managed to outrun the currents. In either case, this water became the river that was known henceforth as the Boyne, and Boann thereafter became the presiding deity.

Another aspect of the myth of Boann is that she bore Oenghus. She and the leader of the Tuatha De Danaan, the Dagdha, engaged in an illicit affair that resulted in the birth of this god of love. However, since both Boann and the Dagdha wished to keep their rendezvous a secret, they used their divine powers to cause the nine month gestation period to last but a single day - or so it seemed, for the sun was frozen in the sky for those nine months, never setting and never rising. On this magical day, Oenghus emerged into the world.
FROM: Mythography | The Celtic Goddess Boann in Myth and Art _____________________________________________________________

Boann/Boand
Irish: BOYN--"cow" or "divine cow"
Modern: "Boyne"

Personification of the River Boyne in Ireland, Boann or Boand is the wife of the Dagda1 and mother of Oengus mac ind-Og.
Legend says that the Dagda had a well of wisdom surrounded by nine hazel-nut trees; the nuts fell into the water and were the source of the wisdom. Only the Dagda and his three cup-bearers were allowed to draw water from the well. However, one day Boann was curious and drew the water for herself. The well overflowed, and pursued Boann to the sea. The water became a river, and the river was called the Boann, or Boyne. Some legends say that she was turned into a salmon and swam in the river, and that she is the same as the salmon of wisdom.

As the mother of Oengus, she is then identifiable with Modron/Rhiannon in Welsh mythology. Oengus is often identified with Mabon ap Modron, and as such is identifiable with Pryderi. As such, along with the significance of the Newgrange settlement and that she is the wife of the Dagda, she may be a sort of Irish version of the Grail bearer, or at least mother of the Grail hero.
Moreover, the story of the salmon reminds me of how Aphrodite/Venus and her son Eros/Cupid (with whom Oengus is often identified) changed into fish, and became the constellation Pisces. It is doubtful this is relevant, but associations are funny things. As a river deity, she is similar to (the hypothetical) Danu for the Danube and Sabrina for the Severn.
FROM: Boann
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Boann was a member of the Tuatha Dé Danaan, a race that eventually lived in the Otherworld, though they could pass freely between this world and the Otherworld.
Their many stories will not be told here.
Perhaps later.
Boann was the Goddess of the River Bóinne (Boyne).
She was married to Nechtán.
She was also consort to Elcmar.
She was also consort to the Dagda, know as the Good God.
When the Dadga met Boann, he sent Elcmar on a day-long errand.
When Elcmar left on his errand, the Dadga came to Boann in her bed.
From that union was born Aengus Mac Óg, the Celtic God of Love.
Due to the Dagda's magic, Elcmar's single day lasted nine full months.
Thus Aengus Mac Óg was conceived and born on the same day.
Near the dwelling place of Boann was a secret well which was the source of all knowledge and inspiration.
Nine hazels grew over the well, and dropped their nuts into the fountain.
Five salmon which were in the fountain severed the nuts and sent their husks floating down five streams.
These are the five streams of the senses through which knowledge is obtained.
This is also the well in which the Salmon of Knowledge was spawned.
That salmon swallowed the hazelnut of wisdom, and it was Fionn Mac Cumhaill who eventually ate the flesh of the Salmon of Knowledge.
Another tale for another time.
Boann had been warned by Nechtán not to test the power of the well.
But she was a prideful woman, and so, tempting fate, she walked three times counter-clockwise around the well.
Three waves rose out of the well, one carrying away her thigh, one carrying away her hand, and the third carrying away her eye.
Shamed by her disfigurement, she turned seaward, the waters of the well roaring behind her, until she reached the sea.
Overcome by the force of the waves as she reached the sea, she drowned.
And thus was the River Bóinne born.
FROM: in pursuit of the soul