Merrow (from Irish murúch, Middle Irish murdúchann or murdúchu) is a mermaid or merman in Irish folklore. The term is of Hiberno-English origin.

The merrows supposedly require a magical cap (Irish: cochaillín draíochta; Hiberno-English: cohuleen druith) in order to travel between deep water and dry land.

The merrow-maiden is like the commonly stereotypical mermaid: half-human, a gorgeous woman from waist up, and fish-like waist down, her lower extremity "covered with greenish-tinted scales" (according to O'Hanlon). She has green hair which she fondly grooms with her comb. She exhibits slight webbing between her fingers, a white and delicate film resembling "the skin between egg and shell".

Said to be of "modest, affectionate, gentle, and benevolent disposition," the merrow is believed "capable of attachment to human beings," with reports of inter-marriage. One such mixed marriage took place in Bantry, producing descendants marked by "scaly skin" and "membrane between fingers and toes". But after some "years in succession" they will almost inevitably return to the sea, their "natural instincts" irresistibly overcoming any love-bond they may have formed with their terrestrial family. And to prevent her acting on impulse, her cohuleen druith (or "little magic cap") must be kept "well concealed from his sea-wife".

O'Hanlon mentioned that a merrow may leave her outer skin behind in order to transform into other beings "more magical and beauteous", But in Croker's book, this characteristic isn't ascribed to the merrow but to the merwife of Shetlandic and Faroese lore, said to shed their seal-skins to shapeshift between human form and a seal's guise (i.e., the selkie and its counterpart, the kópakona). Another researcher noted that the Irish merrow's device was her cap "covering her entire body", as opposed to the Scottish Maid-of-the-Wave who had her salmon-skin.

Yeats claimed that merrows come ashore transformed into "little hornless cows". One stymied investigator conjectured this claim to be an extrapolation on Kennedy's statement that sea-cows are attracted to pasture on the meadowland wherever the merrow resided.

Merrow-maidens have also been known to lure young men beneath the waves, where afterwards the men live in an enchanted state. While female merrows were considered to be very beautiful, the mermen were thought to be very ugly. This fact potentially accounted for the merrow's desire to seek out men on the land.

Merrow music is known to be heard coming from the farthest depths of the ocean, yet the sound travels floatingly across the surface. Merrows dance to the music, whether ashore on the strand or upon the wave. While most stories about merrow are about female creatures, a tale about an Irish merman does exist in the form of "The Soul Cages", published in Croker's anthology. In it, a merman captured the souls of drowned sailors and locked them in cages (lobster pot-like objects) under the sea. This tale turned out to be an invented piece of fiction (an adaptation of a German folktale), although Thomas Keightley who acknowledged the fabrication claimed that by sheer coincidence, similar folktales were indeed to be found circulated in areas of Cork and Wicklow.

The male merrow in the story, called Coomara (meaning "sea-hound"), has green hair and teeth, pig-like eyes, a red nose, grows a tail between his scaly legs, and has stubby fin-like arms. Commentators, starting with Croker and echoed by O'Hanlon and Yeats after him, stated categorically that this description fitted male merrows in general, and ugliness ran generally across the entire male populace of its kind, the red nose possibly attributable to their love of brandy.

The merrow which signifies "sea maiden" is an awkward term when applied to the male, but has been in use for a lack of a term in Irish slang for merman. One scholar has insisted the term macamore might be used as the Irish designation for merman, since it means literally "son of the sea", on authority of Patrick Kennedy, though the latter merely glosses macamore as designating local inhabitants of the Wexford coast. Gaelic (Irish) words for mermen are murúch fir "mermaid-man" or fear mara "man of the sea".

Merrows wear a special hat called a cohuleen druith, which enables them to dive beneath the waves. If they lose this cap, it is said that they will lose their power to return beneath the water.

The normalized spelling in Irish is cochaillín draíochta, literally "little magic hood" (cochall "cowl, hood, hooded cloak" + -ín diminutive suffix + gen. of draíocht). This rendering is echoed by Kennedy who glosses this object as "nice little magic cap".

Arriving at a different reconstruction, Croker believed that it denoted a hat in the a particular shape of a matador's "montera", or in less exotic terms, "a strange looking thing like a cocked hat," to quote from the tale "The Lady of Gollerus". A submersible "cocked hat" also figures in the invented merrow-man tale "The Soul Cages".

The notion that the cohuleen druith is a hat "covered with feathers", stated by O'Hanlon and Yeats arises from taking Croker too literally. Croker did point out that the merrow's hat shared something in common with "feather dresses of the ladies" in two Arabian Nights tales. However, he did not mean the merrow's hat had feathers on them. As other commentators have point out, what Croker meant was that both contained the motif of a supernatural woman who is bereft of the article of clothing and is prevented from escaping her captor. This is commonly recognized as the "feather garment" motif in swan maiden-type tales. The cohuleen druith was also considered to be of red color by Yeats, although this is not indicated by his predecessors such as Croker.

An analogue to the "mermaid's cap" is found in an Irish tale of a supernatural wife who emerged from the freshwater Lough Owel in Westmeath, Ireland. She was found to be wearing a salmon-skin cap that glittered in the moonlight. A local farmer captured her and took her to be his bride, bearing him children, but she disappeared after discovering her cap while rummaging in the household. Although this "fairy mistress" is not from the sea, one Celticist identifies her as a muir-óigh (sea-maiden) nevertheless.

The Scottish counterpart to the merrow's cap was a "removable" skin, "like the skin of a salmon, but brighter and more beautiful, and very large", worn by the Maid-of-the-wave.