In A Field Guide to Irish Fairies*,  Bob Curran attempts to identify the fairies with whom one is most likely to run into.  But first, where did fairies come from?
My favourite explanation is that they are "fallen angels, those who sat on the fence during the great rebellion in Heaven and were thrown out for their indecisiveness. . .
"There were not, however, consigned to Hell with Lucifer and his followers, being neither good enough to be saved or bad enough to be lost.  St. Michael, the patron saint of all fairies, interceded with God on their behalf and they were given the dark and remote placesof the earth in which to dwell, well away from human habitation.  Some were granted the depths of the oceans and became merfolk; others were sent to the lands under the earth (this explains the fairies at the bottom of our garden) and became goblins and trolls.  Others were granted the air and became spirits and sheeries.  Others were given the harsh and barren areas of the countryside and became leprechauns and grogochs."

Tomorrow:  The thirteen types of fairies.

Published by Chronicle Books, 1998  http://www.chroniclebooks.com/