The pattern is fixed across Orkney, Shetland and the Hebrides. A man watches seal women dancing on the shore, takes and hides one skin, and marries its owner. She is a good wife and mother for years. She finds the skin, usually through a child who has seen where it is kept, and she is gone into the sea at once. The stories do not present this as betrayal on her part, and they are not neutral about the man.
Folklorists file it with the animal bride tales found from Japan to the Baltic, and the international pattern is real. What the northern isles add is the specific: the skin is a garment, the marriage is legal and productive, and the departure is a return rather than an escape into wildness. Several families in the Hebrides and the north of Scotland claimed seal descent, the MacCodrums of North Uist most famously, and would not hunt them.
There is a long-standing suggestion that the tradition preserves sightings of kayakers from the far north, seen off Orkney in sealskin clothing and remembered as Finn-men. The sightings are recorded and the inference is old, but a tale type this widely distributed does not need a local origin, and the suggestion is best kept as a proposal rather than an explanation.