Leprechauns are a type of fairy, though it's important to note that the fairies of Irish folklore were not cute Disneyfied pixies; they could be lustful, nasty, capricious creatures whose magic might delight you one day and kill you the next if you displeased them.

While leprechauns are mythical beings, a rare type of insulin resistance, sometimes called leprechaunism, is very real.

Leprechaun lore

Leprechauns are often described as wizened, bearded old men dressed in green (early versions were clad in red) and wearing buckled shoes, often with a leather apron. Sometimes they wear a pointed cap or hat and may be smoking a pipe.

In their book "The Element Encyclopedia of Magical Creatures," John and Caitlin Matthews trace leprechaun legends back to eighth-century legends of water spirits called "luchorpán," meaning small body. These sprites eventually merged with a mischievous household fairy said to haunt cellars and drink heavily.

Other researchers say that the word leprechaun may be derived from the Irish leath bhrogan, meaning shoemaker. Indeed, though leprechauns are often associated with riches and gold, in folklore their main vocation is anything but glamorous: they are humble cobblers, or shoemakers. Shoemaking is apparently a lucrative business in the fairy world, since each leprechaun is said to have his own pot of gold, which can often be found at the end of a rainbow.

According to Irish legends, people lucky enough to find a leprechaun and capture him (or, in some stories, steal his magical ring, coin or amulet) can barter his freedom for his treasure. Leprechauns are usually said to be able to grant the person three wishes. But dealing with leprechauns can be a tricky proposition.

A trickster

The leprechaun plays several roles in Irish folklore; he is principally a roguish trickster figure who cannot be trusted and will deceive whenever possible. In her encyclopedia "Spirits, Fairies, Leprechauns, and Goblins," folklorist Carol Rose offers a typical tale of leprechaun trickery "concerning a man who managed to get a leprechaun to show him the bush in the field where his treasure was located. Having no spade [shovel], the man marked the tree with one of his red garters, then kindly released the sprite and went for a spade. Returning almost instantly he found that every one of the numerous trees in the field sported a red garter!"

In the magical world, most spirits, fairies and other creatures have a distinctive sound that is associated with them. Some entities — such as the Irish fairy banshee and the Hispanic spirit La Llorona — are said to emit a mournful wail signifying their presence. In the case of the leprechaun, it's the tap-tap-tapping of his tiny cobbler hammer, driving nails into shoes, that announces they are near.

In his collection of Irish fairy and folk tales, W.B. Yeats offered an 18th-century poem by William Allingham titled "The Lepracaun; Or, Fairy Shoemaker" which describes the sound:

"Lay your ear close to the hill. Do you not catch the tiny clamour, Busy click of an elfin hammer, Voice of the Lepracaun singing shrill As he merrily plies his trade?"

The 1825 publication of a book called "Fairy Legends" seemingly cemented the character of the modern leprechaun: "Since that time leprechauns seem to be entirely male and solitary," they note.

It seems that all leprechauns are not only shoemakers but also old male loners, which makes sense from a cultural standpoint, since that type of fairy is so closely associated with shoemaking, a traditionally male vocation. Though there is something curious about all leprechauns being cobblers (what if they want to be writers, farmers, or doctors?), this designation also fits in well with the traditional folkloric division of labor among fairies.

Leprechauns in popular culture

As with many old legends and traditions, the image and nature of the leprechaun has changed over time and has been updated (and in some cases sanitized) for a modern audience. Lucky the Leprechaun, mascot of the General Mills breakfast cereal Lucky Charms, is probably the best-known fairy of his type. The 1959 Disney movie "Darby O'Gill and the Little People" also influenced how many people think of the wee folk.

On the other end of the spectrum there's the homicidal leprechaun Lubdan in the "Leprechaun" horror/comedy film series (played by "Willow" actor Warwick Davis). For generations, some Irish have been annoyed by leprechauns and the ethnic stereotypes they perpetuate, and for most Americans leprechauns only appear around St. Patrick's Day.

Leprechauns offer a morality tale figure whose fables warn against the folly of trying to get rich quick, take what's not rightfully yours or interfere with "The Good Folk" and other magical creatures. Belief in leprechauns and other fairies was once widespread on the Emerald Isle, and real or not they will continue to amuse and delight us for centuries more.

Genetic birth disorder

Leprechaunism, also known as Donohue syndrome, is an extremely rare disorder characterized by abnormal resistance to insulin. (Some researchers prefer Donohue syndrome because “leprechaunism” may be viewed as pejorative by families, according to the Online Mendelian Inheritance in Man, “an online catalog of human genes and genetic disorders.”)

It is a recessive genetic disorder, which occurs when an individual inherits two copies of an abnormal gene for the same trait, according to the National Organization for Rare Disorders (NORD).

Babies with the disorder are unusually small before and after birth, according to the National Institutes of Health. They experience failure to thrive, which means they have low birth weight and do not gain weight at the expected rate. They often lack muscle mass and may also have very low body fat under the skin.

Characteristics of the syndrome also include abnormally large, low-set and poorly developed ears; a wide, flat nose with upturned nostrils; large, thick lips and a large mouth; and widely spaced, bulging eyes. Affected babies may also have an abnormally small head, or microcephaly. There may be excessive hair growth.

Most affected individuals have a skin condition called acanthosis nigricans, in which certain patches of skin, such as body folds and creases, become thick, dark and velvety.

Donohue syndrome affects the endocrine system, which regulates the secretion of hormones into the blood system. Abnormalities include excessive secretion of insulin, which regulates blood sugar levels by promoting the movement of glucose into the body’s cells. According to NORD, babies with the disorder cannot use insulin effectively and may have high blood sugar levels, or hyperglycemia, after eating and low blood sugar levels, or hypoglycemia, when not eating.

Other hormonal effects include enlargement of the breasts and genitals. Other characteristics include intellectual disability, abnormally large hands and feet, an enlarged or distended stomach, enlarged heart, kidneys and other organs; and hernias, where the large intestine may protrude through the abdominal wall or into the groin. Affected babies are also more susceptible to repeated infections.

Donohue syndrome is extremely rare; only 50 cases have been reported in medical literature. It was first identified in 1948 by Dr. W.L. Donohue, a Canadian pathologist who wrote about it in the Journal of Pediatrics in 1954. In the reported cases, the disorder occurred twice as often in females as in males.

Treatment is usually directed toward the specific symptoms, according to NORD. Endocrinologists treat the hormonal issues, while dermatologists treat the skin problems, for example. Families may also receive genetic counseling.

Benjamin Radford is deputy editor of Skeptical Inquirer science magazine and author of six books, including "Tracking the Chupacabra: The Vampire Beast in Fact, Fiction, and Folklore." His website is www.BenjaminRadford.com.

Additional reporting by Reference Editor Tim Sharp. Follow him on Twitter @TimothyASharp