Ruthie from the Celtic Myth Podshow introduces us to these underwater faeries.

The word merrow or moruadh comes from the Irish muir (meaning sea) and oigh (meaning maid) and refers specifically to the female of the species. Mermen – the merrows male counterparts – have been rarely seen. They have been described as exceptionally ugly and scaled, with pig-like features and long, pointed teeth. Merrows themselves are extremely beautiful and are promiscuous in their relations with mortals.

The Irish merrow differs physically from humans in that her feet are flatter than those of a mortal and her hands have a thin webbing between the fingers. It should not be assumed that merrows are kindly and well-disposed towards mortals. As members of the sidhe, or Irish fairy world, the inhabitants of Tir fo Thoinn (the Land beneath the Waves) have a natural antipathy towards humans. In some parts of Ireland, they are regarded as messengers of doom and death.

Ruthie goes on to equate merrows with selkies, women who take on the form of seals by wearing a magical seal skin. To my mind, these are separate creatures, though it is certainly true that legends tend to be fluid over time and distance. There are certainly points of overlap between them.