The Leannan Sidhe

What is the Leanan Sidhe? You may have encountered descriptions of this creature as a type of female fairy which grants inspiration to male poets, but drains the life and vitality from them, like a vampire muse. However, this version comes directly from the work of the poet W. B. Yeats, who could be . . . creative with his use of folklore. 

Here's what Yeats had to say in his 1888 book Fairy and Folk Tales of the Irish Peasantry.
The Leanhaun Shee (fairy mistress), seeks the love of mortals. If they refuse, she must be their slave; if they consent, they are hers, and can only escape by finding another to take their place. The fairy lives on their life, and they waste away. Death is no escape from her. She is the Gaelic muse, for she gives inspiration to those she persecutes. The Gaelic poets die young, for she is restless, and will not let them remain long on earth—this malignant phantom.
And:
...the Lianhaun shee lives upon the vitals of its chosen, and they waste and die. She is of the dreadful solitary fairies. To her have belonged the greatest of the Irish poets, from Oisin down to the last century.

Most mentions of the Leannan Sidhe since then (under various spellings) draw directly from Yeats' description of a vampire-like seductress. His account bears a strong resemblance to concepts like John Keats' 1819 poem "La Belle Dame sans Merci."

But Yeats' version doesn't necessarily coincide with the original Irish concept of this being.

A year before Yeats' book came out, Lady Jane Wilde described a significantly more benevolent version of the same creature. "The Leanan-Sidhe, or the spirit of life, was supposed to be the inspirer of the poet and singer, as the Ban-Sidhe was the spirit of death, the foreteller of doom. The Leanan-Sidhe sometimes took the form of a woman, who gave men valour and strength in the battle by her songs." Wilde lists Eodain the poetess as a Leanan-Sidhe. (Ancient Legends, Mystic Charms and Superstitions of Ireland, 1887)

And this being was not always female. In the 1850s, the Transactions of the Ossianic Society spent quite some time laying out details of the "Leannan Sighe" as muses who would inspire bards, only to take them away after a shortened lifespan. The most interesting piece of evidence is an incantation from 1760, created to expel a male Leannan Sighe. Like an incubus, he was harassing a local woman named Sheela Tavish. The incantation, composed by a Catholic priest, is an odd mix of religions and mythologies. At one point he appeals to the fairy queen Aoibheall. The writers of the article suggested that the poem was satirical. They also mentioned meeting "many persons who pretended to be favored with the inspirations of a Leannan Sighe," these apparently being trick psychics.

Yeats' inspirations can be clearly seen here, but it is also clear that he has cherry-picked details.​ Most pertinently, the Leannan Sighe in traditional Irish lore can be male or female, but Yeats' version is always female.

The Lianhaun Shee is mentioned in John O'Hanlon's Irish Folk Lore (1870) and in The Journal of Science (1872). In Irish Folk Lore, there's reference to the being's fluid gender and to its habit of aiding men in battle (which apparently extends to bar fights). 

In the 20th century, the Irish Folklore Commission collected an account of a healer named Eibhlin Ni Ghuinniola who was sometimes seen gathering herbs in the company of a male leannan si. (Crualaoich p. 189-191)

Fairy lovers could be helpful or harmful. In the myths of Fionn mac Cumhaill, Uchtdealbh was a jealous fairy who cursed her lover's wife. She would have been described as some variation of Leannan Sighe since that's literally what she was - a fairy lover. Biroge of the Mountain was a more benevolent spirit who aided the hero Cian.

The New English-Irish Dictionary defines the leannan si as "fairy, phantom, lover," "baleful influence," or "sickly complaining person."

The Lunantishee
There’s one odd side note in Evans-Wentz’s Fairy Faith in Celtic Countries (1902), among material collected from a tailor named Patrick Waters.
"The lunantishees are the tribes that guard the blackthorn trees or sloes; they let you cut no stick on the eleventh of November (the original November Day), or on the eleventh of May (the original May Day). If at such a time you cut a blackthorn, some misfortune will come to you." (p. 53)

This may make more sense when we recognize that several of Waters’ descriptions of fairy races were atypical. He described pookas as horse-dealers who invisibly visited racecourses, and referred to the “gentry,” or fairies, as coming from “the planets—according to my idea.” (So . . . aliens?) Evans-Wentz also seemed mystified by some of Waters’ accounts, including one Druid story that he was “unable to verify in any way” (p. 52).

The lunantishee is a little baffling, but comparing it to the many spellings of Leanan Sidhe, I think it's a version of the same name. If pookas are horse-dealers and the Gentry are aliens, then some shuffled attributes aren't that out-of-place.

The connection between fairies, blackthorns and sloes has some possible evidence. An 1894 edition of Transactions of the Folk-lore Society mentioned that the "good people" protect solitary bushes, and mentions specifically that it is unlucky to cut down a lone blackthorn bush. Similar plants like the white-thorn and hawthorn were also sacred to the fairies, in Ireland but also in other places such as Brittany. (Thiselton-Dyer, 1889)

In 1907, Hugh James Byrne mentioned that in Connaught "the fairies are supposed to blight the sloes and haws and other berries on November night." From The Folk-Lore Record in 1881 - in North Ireland it's said that "On Michaelmas Day [September-October] the devil puts his foot on the blackberries." Thomas Keightley mentions in The Fairy Mythology that when blackberries begin to decay, children are warned "not to eat them any longer, as the Pooka has dirtied on them." It's a cautionary story, using the threat of fairies to stop children from potentially eating spoiled berries. Similarly, Churn-Milk Peg was a British spirit who punished those who ate underripe nuts. 

Conclusion
Both Yeats' Leanhaun Shee and Patrick Waters' lunantishee appear to be unique renditions of an Irish creature which is both complex and . . . actually rather simple. A fairy lover is a fairy who loves a human, who may be male or female, who may aid or harass humans. Yeats synthesized this into a species of vampiress. Waters, on the other hand, juggled different fairy traits and perhaps his own imagination, giving a hint of what fairy belief might have looked like around 1900 or so. 

Further Reading

• Crualaoich, Gearoid. The Book of the Cailleach: Stories of the Wise‑woman. 2003.
• Evans-Wentz, W. Y. The Fairy-faith in Celtic Countries. 1911.