Merrows are practically regular mermaids in the Irish folklore, despite the fact that they are more of a race of sea-beings. they are often described as having green, or white skin, and sea-colored hair. They have webbed fingers, and the same type of tail as a mermaid. In some legends, it is said that they carry either a red, feathered cap; a red cape; or a belt that enables them to dive back down to the deep water after surfacing. But, if a human were to steal the merrow's item, they would not be able to go back down in the water. The human would often hide the object.
This is where their offspring come in. When a human steals the merrow's belongings, the human most likely does it to mate with the merrow. The offspring result in having webbed fingers, and webbed toes, but otherwise: human.
When a merrow finds her stolen object, she uncontrollably feels the call of the ocean and goes back home.
The male Merrows, on the other hand, are horribly ugly with a red bulby nose, green skin, and a displeasing complexion (this is often why merrows sometimes purposefully go to the surface to mate with human men).
Faeries by Brian Froud