Probably the oldest examples we have of these stories come from mythology, although I admit it gets murky to delve into this in the Irish were the Gods and the aos sidhe are often only thinly divided. We have the story of Niamh and Oisín, where Niamh is usually described as a woman of Fairy although she can equally be called a Goddess as a daughter of Manannan. Although it involves reincarnation and a Goddess reborn as a human, we also have the story of Midhir and Etain, where Etain is born as a human girl and is courted and won by the fairy King/God Midhir. There is Áine, who we know is a Goddess but is also a fairy Queen, and who is the progenitor of the FitzGeralds; she took the Earl of Desmond as her lover and gave him a son, Geirriod. In a similar vein there is the McCarthy family who are said to be descended from Cliodhna - Goddess and fairy Queen. In the second two stories there are overtones of the sovereignty Goddess marrying a mortal lord to legitimize his rule, but in all the stories we see an Otherworldly being taking a mortal as a lover and in three of the tales having children with them. We could also add the conception of Cu Chulainn to this, although it is a bit more metaphysical in some versions, as we see Lugh - again a God and one of the aos sidhe at this point - coming to Deichtine either in reality or in a dream and fathering Cu Chulainn on her.
|'La Belle Dame sans Merci'|
Beyond the mythology we also see many examples in older folklore. There were several types of fairies specifically known for seducing mortals, including the aforementioned Leannán Sí as well as her male counterpart the Gean-cánach; these generally did so to the mortal's ultimate detriment. However stories of mortals having sexual relationships with fairies, often producing children, are found across fairylore and with a wide array of types of fairies including kelpies, selkies, aos sidhe, and lake maidens. In the kelpie lore the kelpie can be male or female and while kelpies are more known for tricking and harming people in these cases the kelpie falls in love with the mortal and seduces them. Sometimes the mortal awakes after a tryst and sees their sleeping lover only to notice the telltale bit of water-weed in their hair, or dripping water, or other give-away sign that reveals their nature and the mortal flees. Other times the two wed and only after a child is born does the mortal realize their spouse's true nature and leave; although their is one iteration of the story where a male kelpie captures and imprisons a mortal girl as his 'wife' and she escapes after a year, usually leaving behind a son. In the selkies tales the male selkie woos the mortal girl to his home under the waves, while the female is only taken as a bride to a mortal - in the stories - if her sealskin coat is taken from her. Again however children are the usual result of the marriage. You can see the pattern here. The aos sidhe stories appear under a variety of forms usually with the human being kidnapped or taken into Fairy, sometimes willingly sometimes not. The lake maidens of Wales usually are willing wives but come with geasa, and once those taboos are broken they immediately leave, like the selkie bride finding her sealskin, returning to the waters they came from.
Some versions of the story of the MacLeod Fairy Flag say it was a gift to the family from a fairy lover who had born a MacLeod child. One of the most widespread stories found in fairylore across different Celtic cultures is that of the borrowed midwife who anoints the baby's eyes and accidently touches her own, only to be granted true sight and realize that she has delivered the half-fairy child of a local girl*. This girl has been taken into Fairy as the wife of one of its inhabitants and obviously just proven that humans and fairies are in fact cross-fertile. Which shouldn't be surprising since one of the leading theories about changelings is that fairies steal people to supplement their own population, and that doesn't exactly mean the people become fairies so much as it means the people make more fairies, as the midwife tales illustrate. The gender-flip version of this story might be the Ballad of Tam Lin although Tam Lin is more properly a Changeling as opposed to being a true fairy himself; in the story however the mortal girl, Janet, does not know that until after she's taken him for her lover and conceived his child. Only when she goes to the well he guards to gather herbs to abort the pregnancy and Tam Lin appears to stop her does she question whether he was ever human**. Many of the Scottish witches who confessed to dealing with the fairies also admitted having sexual relations with them, as opposed to the more usual demonic intercourse other witches admitted to. At least one 19th century Bean Feasa was known to have had a fairy lover as well.
In the book 'The Good People', which is an anthology of collected articles about fairies there are several discussions of more modern anecdotes. These come from interviews with people in Ireland, Wales, and Scotland conducted in the 20th century and looking at modern fairy beliefs. It is, in its own way, the next generation of Evans-Wentz's 'Fairy Faith in Celtic Countries' and it includes some discussion of fairy lovers and of the children born of these unions. Generally to have a fairy lover carried prohibitions (geasa) often of silence about their existence but sometimes it might be something like not striking the spouse three times. With the selkies, who were unwilling brides, the magical sealskin must be hidden, and in Welsh lore a fairy wife was often secured by learning her True Name which had power over her. The children of these unions were known to be uncanny and in many stories were taken into Fairy by their Otherworldly parent; those who remained in our world generally stood out as odd or unusual in their mannerisms and preferences; like their fairy parent they tended to behave in ways that seemed to defy human mores or etiquette as often as not. Children born with selkie heritage were said to long for the sea and often to have webbed hands or feet, as well as dark hair and eyes.
So, we can see that there's a long established pattern of fairies taking human lovers. Sometimes only as lovers, sometimes as spouses, sometimes producing children, sometimes not. Usually the human half of this equation is someone who has broken social boundaries by seeking the fairies out, such as we see in stories of woman going to do their work in places known to belong to Themselves or going to wells known to be Theirs, or of people who are in a liminal state, for example about to be married. Keep in mind as well this wasn't, for the most part, figurative or imagined - not 'on the astral' as some people might say - but occured in the physical, tangible world. Other people reported seeing these beings in some cases and the resulting children were real, physical children. Usually when the person was taken into Fairy they were thought to have died, which in the parlance of Fairy means they may have actually died in our world. Give that some thought.
Before you go rushing out to find a fairy lover of your own it is worth considering that as often as not these things end badly. And by badly I mean with the death of the mortal partner, sometimes through mischance and sometimes through violence. In other stories the mortal violates a taboo - a geis - set down by the fairy partner and loses them forever, which generally drives the mortal mad. So this whole concept is a bit more dangerous than your average Tinder hook-up, and shouldn't be treated lightly.
*in different versions the girl was either thought to be dead, missing, or known to be taken by fairies. In all versions the midwife later sees the husband at a fair and he puts out her eye when he realizes she can see him.
**Oh Janet, really? Seems like the sort of question you might have wanted to ask a smidge earlier, if it mattered. Like before you lifted your skirt.
Briggs, K., (1976). A Dictionary of Fairies
Briggs, K., (1967). The Fairies in Tradition and Literature
Silver, C., (1999). Strange and Secret Peoples: Fairies and Victorian Consciousness
Purkiss, D., (2000). At The Bottom of the Garden: A Dark History of Fairies, Hobgoblins, and Other Troublesome Things
Narvez, P., (1999) The Good People