In Irish folklore, a mermaid-like fairy is called a merrow.
The English word “merrow” is an anglicised form of the Irish murúch and is used for a mermaid or merman in Irish tradition. The creature became especially prominent in nineteenth-century collections of Irish folklore, including Thomas Crofton Croker’s Fairy Legends and Traditions of the South of Ireland, first published in 1828.
Folkloric descriptions often portray merrow-maidens as beautiful, green-haired beings with fish-like lower bodies and a gentle or affectionate disposition. Some stories say they can marry humans. They are also associated with a magical cap, called a cohuleen druith, which allows them to move between the sea and the land; a human who hides the cap may prevent a merrow from returning underwater.
Merrow is not simply another word for a selkie. Selkies are seal-people whose transformation depends on a seal skin, whereas merrows are associated with mermaid imagery. Irish tradition also preserves older terms such as murdúchann, a siren-like being in medieval literature, so “merrow” is the standard answer for this particular folklore category rather than the only historical Irish word connected with sea beings.