The mer-people, or Merrow as they are called in Ireland, live under the ocean in a splendid kingdom. They are human from the waist up, but the rest of their body is a fish. They are beautiful beings, who often consort with mortals, and sometimes these unions bring about children.In fact, some families claim their lineage from such a coupling.
Another race of sea-dwelling beings are the Selkie, humans on land who return to their seal form when they return to the ocean by wearing a magical cap. To steal the cap renders the selkie powerless to return to his or her home. Usually this is done by a human captivated with the selkie's beauty. The selkie then will become a devoted partner...but should the magical cap ever be found, the selkie will leave the human for his or her home in the sea, usually leaving the mortal to die from grief. Serves the mortal right, I say!