There is an Irish quasi-historical myth in which a foreign king comes by the sea and lands in Ireland where he encounters a princess and they dally for one afternoon and he tells her, "You shall be born of a king" and he gives her his ring as a momento and tells her to give the ring to her child if it should be a son, then departs. Eventually she gives birth to an important king. What is this myth?
In the story the princess rejects many suitors, but when she meets the strange prince from the sea, he says to her "Shall we have an hour of lovemaking?" and she agrees and asks "How is that all the princes of my land have asked for my hand in marriage, but you possess me so?" Immediately after the encounter he departs so they are together only for an afternoon.
Note that in the Irish histories, this story is represented as a true story and the descendants of this supposed union are actual high kings of Ireland.