Why did the Irish believe in leprechauns?

The myth probably goes back to the story of where there is a pot of gold under the rainbow, and that would be where you'd see a leprechaun. And it might symbolize inherent positivity, despite the natural shortness of some humans' body's and red hair; and and those of us who are from that country that has a fiddle-dee-dee accent, whereas the majority of Dubliners have a Colin Farrellesque accent.