The Element Encyclopedia of Witchcraft: The Complete A-Z for the Entire Magical World - Judika Illes 2005
Fairy forts, also known as ring forts or stone forts, are roughly circular earthen banks or stone walls. There were once as many as 60,000 of these circular earthworks in Ireland. Local names for them include cashel, forth, rath or rusheen.
The majority of them appear to have been built as enclosures for dwellings in the latter half of the first millennium of the Common Era, and they are classified as archeological remains of early medieval dwellings, now long deserted.
Ring forts became known as fairy forts because allegedly they are among the favorite haunts of fairies. According to common international metaphysical wisdom, ruins attract spirits: in North Africa and the Middle East, children are advised to avoid ruins so as to avoid encountering Djinn, just as Irish children were taught to stay out of ring forts for fear of fairies.
Of course, those who wish to encounter the spirits know where to go to find them. Being observed exploring a ruin such as a ring fort was most often interpreted as attempting to contact these spirits and was actively discouraged, although those who wished to commune with fairies persisted in this practice.
Ring forts are usually overgrown with vegetation. They are wild, mysterious places, considered the homes of spirits, the haunts of fairies. Ring forts can be physically as well as spiritually perilous, as many contain underground passages.
See also Fairy Faith, Fairy Witch.