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Fairy forts (also known as raths from the Irish, referring to an earthen mound) are the remains of lios (ringforts), hillforts or other circular dwellings in Ireland. From (possibly) late Iron Age to early Christian times, the island's occupants built circular structures with earth banks or ditches. These were sometimes topped with wooden palisades, and wooden framed buildings. As the dwellings were not durable, in many cases only vague circular marks remain in the landscape. Raths and lios are found in all parts of Ireland.
Tradition claimed that ringforts were "fairy forts" imbued with Druids' magic and believers in the fairies did not alter them. The early pre-Celtic inhabitants of Ireland (known as the Tuatha Dé Danann and Fir Bolg) came to be seen as mythical and were associated with stories of fairies, also known as the "Good People". Fairy forts and prehistoric Tumuli were seen as entrances to their world. Even cutting brush, especially the sceach or whitethorn, on fairy forts was reputed to be the death of those who performed the act.
There are many folk tales about supernatural events happening at fairy forts. Real accidents which happened at ringforts could be given supernatural explanations. For example a man who tried to blast a dolmen suffered a septic hand. The wrecked dolmen was subsequently left untouched.
A cow taken and restored
A farmer’s best cow kept grazing in a fairy fort. It was unlucky for the cow to graze there but the cow pined when it was prevented from going to the fairy fort. One day the farmer found the cow there with broken legs. He killed the cow and his family ate the meat, some fresh and some salted. A year later the cow was seen in the fairy fort. The fairies told the farmer they had taken the cow because they needed the milk for their children. They had substituted an old stray horse and made the horse to be like the cow. The farmer took his cow home. He became very prosperous because the fairies supported him. 
An old fairy was prevented from marrying a young girl
A rich farmer’s son investigated why none of the cows would enter a field with a fairy fort and found an old fairy in the fort. The old fairy asked the young man to help him get a young girl for his wife. The farmer’s son would not give the young girl to the old fairy but instead married her himself. As revenge the old fairy destroyed most of the father’s property.
The farmer’s son and his wife rode to her parents’ house. The daughter proved who she was. The daughter had three brothers. The brothers went to the fairy fort and started digging till they found a large flat stone. The old fairy begged them to spare his house. When they spared it he became their friend and restored what he had taken. 
Developer's downfall is fairies’ revenge
- Fairy Forts, Music & Language of Ireland
- The Celts & Celtic Ireland[dead link]
- Myths, Legends, Fantasy... - An Other World
- Eddie Lenihan and Carolyn Eve Green, Meeting The Other Crowd: The Fairy Stories of Hidden Ireland, p 125 ISBN 1-58542-206-1
- "A Folklore Survey of County Clare: Fairies and Fairy Forts and Mounds". Clarelibrary.ie. Retrieved 2014-03-13.
- "Ireland". Annl_1.tripod.com. Retrieved 2014-03-13.
- http://www.clarelibrary.ie/eolas/coclare/folklore/folklore_survey/chapter4.htm http://www.clarelibrary.ie]
- Greg Harkin (2012-12-04). "Sean Quinn’s downfall is fairies’ revenge say locals in Cavan". Independent.ie. Retrieved 2014-03-13.
- Celtic Ireland 2000 BC - 400 AD
- Clare Library - Folklore Survey - Fairies and Fairy Forts and Mounds
- Irish Fairy Tales