The leprechaun according to folkloree
Leprechauns are a class of “faerie folk” associated in Irish mythology 
and folklore with the Tuatha_Dé_Danann and other quasi-historical races said to have inhabited Ireland before the arrival of the Celts. Leprechauns and other creatures of Irish mythology are often associated with “faerie forts” or “faerie rings” — often the sites of ancient (Celtic or pre-Celtic) earthworks).
The name “leprechaun” comes from the Irish Gaelic word luprachán, meaning “half-bodied”: like other mythological races in the Irish tradition, leprechauns are considered to be partly real, physical, creatures and partly spirits.
Movies, television cartoons, and commericals for the breakfast cereal Lucky Charms have popularized a specific image of leprechauns which bears scant resemblance to anything found in the cycles of Irish mythology.
The leprechaun in modern popular culture
The modern image of a leprechaun is almost invariant: a diminutive old man, usually no larger than three feet tall, wearing a cocked hat called a tam o’ shanter, leather (work) apron, woolen waistcoat, knee breeches, long stockings and silver-buckled brogues. They are always bearded and are usually pipe smokers and shoemakers. Leprechauns are often depicted wearing emerald green frock coats.
Other features popularly associated with leprechauns are the knowlege of the location of buried treasure, often in a crock of gold.
Many Irish people find the popularized image of a leprechaun to be little more than a series of offensive Irish stereotypes and a trivialisation of Ireland’s rich and ancient Irish mythology.
Info From http://en.wikipedia.org/wiki/Leprechaun