As the man approached, they stopped dancing and raced for the seal-skins. Putting them on they plunged into the sea in the form of seals. In the confusion that they had left one skin behind, which was lying at his feet. He snatched it up, ran to his home and hid it.
On returning to the shore, he met the fairest maiden that he had ever seen. She was walking up and down, moaning about the loss of her seal-skin robe. Without it she could never rejoin her family and friends below the waters. She would have to remain in the world lit by the sun.
The man approached her and tried to console her. She begged him to restore her dress, but the sight of her lovely face, more beautiful in tears, had steeled his heart. Mermaids are always beautiful but never more so than in Celtic myth. He told her that it was impossible for her to return, and that her friends would soon forget about her. Finally, he asked her to marry him.
The sea-maiden, finding she had no alternative, at length consented to become his wife. Marriage is easily arranged in Celtic myth, and they lived together for many years. During that time they had several children, who had no sign of their marine origin except for a thin web between their fingers and a bend of their hands, like the fore paws of a seal. Characteristics that remain in their family to this day.
The Shetlander loved his beautiful wife, but she did not return to his affection. Often she would sneak out alone and hasten down to the lonely beach. There a large seal would appear, and they would converse for hours in an unknown language. She would return home from these meetings thoughtful and sad.
According to this Celtic myth, many years then glided away, and her hopes of leaving the upper world had nearly vanished. One day, one of the children, playing behind a stack of corn, found a seal-skin. Delighted with his prize, he ran with it to his mother. Her eyes glistened with delight because she knew at once that it was her own dress. She was free at last to return to her friends beneath the waves. One thing alone troubled her. She loved her children and she was now about to leave them forever. After kissing and embracing them several times, she took up the skin and proceeded down to the beach.
A few minutes later her husband came in, and the children told him what had occurred. The truth instantly flashed across his mind, and he hurried down to the shore with all the speed that love and anxiety could give. But he only arrived in time to see his wife take the form of a seal and plunge into the sea.
The large seal, with whom she used to converse, congratulated her on her escape. But before she vanished she turned round to her husband, who stood in despair on the beach. "Farewell," said she, "and may all good fortune attend you. I loved you while I was with you, but I always loved my first husband better."
Of all the versions of this Celtic myth that I have read this one has the best ending.
Celtic Myth --> Mermaid Mythology