“Breton heroine or goddess” … “Only Dahut had the key to the gates that kept back the waters, and she wore it on a chain around her fair neck.” … “That she brings death as well as life to the city suggests that she was based on a misinterpretation of an ancient divinity of life’s cycle.”
— Entry “Dahut”, The Encyclopedia of Celtic Mythology and Folklore, Patricia Monaghan. Infobase Publishing, 2004.
“Dahut wore the key to these gates around her neck, for opening them would flood the city.”
— Entry “Dahut”, Encyclopedia of Goddesses and Heroines, Patricia Monaghan, ABC-CLIO, 2009.
“In Christianized treatments of the legend, Dahut may be known as Keben…”
–Entry “Dahut”, A Dictionary of Celtic Mythology, James MacKillop, Oxford University Press, 2004
All of the italicized statements are either misleading or flat wrong. Dahut is never presented as either heroine or goddess in any actual version of the legend, but always as culpable for the destruction of Ys; in many versions she also murders a young man every morning. Only modern fiction of the last few decades have altered this to present Dahut in a more positive light. (Oddly, this same book’s entry “Ys” mentions Dahut’s “living a life that was both sensual and rapacious, killing men after she had her way with them.” The inconsistency with the description of Dahut as a ‘heroine’ is completely unremarked.)
I do not entirely rule out that some obscure telling of the legend has Dahut as keeper of the key – though it seems unlikely. In all the common versions it is her father the king, Gradlon, and even if the author had found some counterexample, a reference work is at least irresponsible to leave this out.
The comment about a divinity is utterly baseless. There is no hint anywhere in the development of the Ys legend that Dahut ever played any role different from that in the mature legend; the earliest mention of Dahut (appearing in Vie des Saincts de la Bretagne Armorique in 1637, by Albert Le Grand) has all the essential touches of the fuller 19th century forms. Indeed, connecting Dahut to an (otherwise entirely unattested) goddess requires postulating at least 1000 years of survival of this goddess-tradition with no evidence appearing at any point. On the other hand, Dahut fits quite well as a simple example of debauchery or sin in an essentially Christian tale.
Similarly, there is no such thing as a “Christianized treatment” of the story since it is essentially Christian from the start, appearing first in a saint’s life!
(The similar figure Mererid in the Welsh sunken-land story of Cantre’r Gwaelod or Maes Gwyddno, on the other hand, may have pre-Christian origins — though this is far from proven. But the similarities of Mererid and Dahut may be due to borrowing rather than common origin anyway.)