Where Lagan stream sings lullaby
There blows a lily fair
The twilight gleam is in her eye
The night is on her hair
And like a lovesick leanan-sidhe
She has my heart in trall
Nor life I own nor liberty
For love is lord of all

And often when the beetle's horn
Hath lulled the eve to sleep
I steal unto her shieling lorn
And thru the dooring peep
There on the cricket's singing stone
She spares the bogwood fire
And hums in sad sweet undertone
The song of heart's desire

To quote Alice Flynn who in turn quotes from Mary O'Hara's notes on this song, in her book "A Song For Ireland", -

"The leanan sidhe (fairy mistress) mentioned in the song is a malicious figure who frequently crops up in Gaelic love stories. One could call her the femme fatale of Gaelic folklore. She sought the love of men; if they refused, she became their slave, but if they consented, they became her slaves and could only escape by finding another to take their place. She fed off them so her lovers gradually wasted away - a common enough theme in Gaelic medieval poetry, which often saw love as a kind of sickness. Most Gaelic poets in the past had their leanan sidhe to give them inspiration. This malignant fairy was for them a sort of Gaelic muse. On the other hand, the crickets mentioned in the song are a sign of good luck and their sound on the hearth a good omen. It was the custom of newly-married couples about to set up home to bring crickets from the hearths of their parents' house and place them in the new hearth."

This mythological femme fatale reminds me of the vilia of Germany, used in the song by Franz Lehar in The Merry Widow opera. Vilia, the spirit woman of the wood, entices the huntsman, and if he sees her he falls in love, which means his death. "vilia, oh, vilia, be tender be true, love me and I'll die for you."
Alice Flynn