Wiktionary
n. The remains of a ringfort or other circular dwelling, superstitiously said to be associated with fairy or leprechauns.
Wikipedia
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.
Usage examples of "fairy fort".
Time in a fairy fort moves at a rapid rate, and a sidereal year was eight years in the life of Dhrun.
She was inside a camouflaged mound of earth, commonly known as a fairy fort.