Editing Diarmuid Ua Duibhne (section)

===King Under-Wave===

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One snowy winter night an unkempt and repugnant woman, "very wild and ugly", appears at the lodge of the Fianna and requests to share one of the men's bedsteads. She is rejected by all but Diarmuid. She makes additional demands on the group's hospitality, which Diarmuid points out are impertinent given her appearance but grants regardless.{{sfn|Gregory|Yeats|2012|p=381}} In the morning, the visitor has magically become young and beautiful, and a grand new house has magically appeared on a nearby hill, "ready for them, with food and servants; and everything they could wish for." Overjoyed, Diarmuid asks the woman to move into the new house with him. She agrees on one condition: he cannot say out loud how ugly she looked on the night they met, at least not three times.{{sfn|Gregory|Yeats|2012|p=382}}

After three days in his new house, Diarmuid grows restless. The woman encourages him to join his comrades for the day. She promises to take good care of Diarmuid's beloved [[greyhound]] and her three new pups. While Diarmuid spends the day hunting, however, one of the Fianna manipulates the woman into giving him one of the pups. Returning home, Diarmuid gets upset and mentions the repellent state the woman arrived in.{{sfn|Gregory|Yeats|2012|pp=382–383}} The following two days, the Fianna convince the woman to give away the remaining two pups, and Diarmuid again brings up her former ugliness. The third time he does so, the woman and the house disappear, and the greyhound dies. Carrying his dead dog, Diarmuid sets out to search for his lady.{{sfn|Gregory|Yeats|2012|pp=383–384}}

An enchanted ship carries Diarmuid "out over the sea, and then down below it", to the otherworldly "Land-under-Waves". Walking its plains, he finds and collects three drops of blood. He also learns that the daughter of King Under-Wave has just returned home from abroad. She has been under some form of enchantment for seven years and is now gravely ill, beyond the help of physicians.{{sfn|Gregory|Yeats|2012|p=385}} The daughter turns out to be Diarmuid's lady. She is overjoyed to see Diarmuid again but states that she will never be well again – partly because of the drop of blood from her heart she lost every time she thought of him on her way home, partly because the cure for her illness consists in three draughts from
the cup of the King of Magh an Ionganaidh, the faraway Plain of Wonder. Diarmuid leaves to retrieve the cup.{{sfn|Gregory|Yeats|2012|p=386}}

With the assistance of a mysterious "low-sized, reddish man", Diarmuid reaches the [[dun (fortification)|dun]] of the King of the Plain of Wonder and demands the cup. The kings sends multiple waves of fighting men, whom Diarmuid slays in many hours of battle. Having lost his army, the king himself emerges. Having learned that Diarmuid is a man of the Fianna of Ireland, the king realizes that Diarmuid is fulfilling a prophecy and willingly parts with his cup.{{sfn|Gregory|Yeats|2012|pp=386–387}} 

The "red man" instructs Diarmuid in the use of the cup but warns him that, along with her sickness, Diarmuid's love for the King's daughter will be gone.{{sfn|Gregory|Yeats|2012|pp=387–388}} He identifies himself as a messenger from beyond the world who has come to Diarmuid's help because Diarmuid's "own heart is hot to come to the help of another". He further warns Diarmuid not to accept the "great riches" the King will offer him for healing his daughter but to ask only for a ship to bring him home to Ireland. The red man's prediction turns out to be correct. Diarmuid departs.{{sfn|Gregory|Yeats|2012|p=388}} In some versions of the story, the King's daughter restored the greyhound to life as a farewell gift.{{citation needed|date=February 2024}}