The drowning of Boann

This legend tells of the temptation of Boann and her drowning at Inbher Colpa. The legend is quoted almost verbatim from a folklorist who lives in Drogheda, located near the mouth of the Boyne.

The Tuatha De Danann

The legend of the hound was that Boann was very curious and there was nobody allowed to go near the sacred well except her husband and his cup-bearers.
She got curious and went to the well with her dog, Dabilla. She lifted the cover off the top of the well - it was a lid or cover of some kind - no-one was allowed to touch it except the king (Nechtain) and his cup-bearers.
The water rose up and covered them up and swept them from there, 70 miles all the way to the sea at Baltray. The well is located at Carbury, a small village in County Kildare, where it can still be seen today.
The water swept Boann and her dog out to sea at Baltray.
Inver (Inbher or Inber in proper Gaelic) Colpa is the old name for Baltray. Colpa was one of the sons of Milesius. He was drowned at the Boyne where it enters the sea at Inbher Colpa.
There's a small mound there near the Protestant Church in Colpe, which has retained its name from this legend.
Inver Colpa means the Inlet of Colpa.
Boann, the Queen, was married to Nechtain, who at that time was the king of Leinster.
It was only him and his cup-bearers, or his closest conifants, probably druids, who were allowed near the well.
It is said that if you look out at Rockabill, it has the shape of a big hound. Dabilla was the name of the dog.
In the name Rockabill, the BILL part commemorates this hound. If you look at the shape of it from a certain angle it's like a massive big hound. That (Rockabill) is presumably the modern remains of the dog.
That is a very old legend, one of the original legends of this area, and it makes sense, because all the original names are still there. The whole thing fits well together.
The well in Carbury is still there to this day and I have seen it.

This version of the legend was submitted to Mythical Ireland by Chris Siren:

In Pagan Celtic Britain by Anne Ross, a less detailed account of the legend appears. Boand, whose name means "She who has white cows", defied the Well of Segais, a spring sacred to king Nechtan. Because of this act of defiance, the waters rise up in anger and chase Boand to lay vengeance upon her. Through their chase to the sea, the waters form the river Boyne, and as they catch her, they mutilate and drown her. Ross also draws a parallel between Boand and Sinann, the eponymous goddess of the River Shannon, who died through similar means. (Ross pp. 47, 279, 280-281). This may not be particularly more detailed than the account on the above web page, but perhaps Ross's sources are.