Knockfierna became famous for the appearance of a pair of leprechauns in the autumn of 1938. Newspaper headings around the world announcing the news that 'Fairies appear in county Limerick' causing thousands of people to make their way up the hill in the hope of meeting the little people. The hill was also the site of an ancient Lughnasa assembly and associated with this was the tradition to visit the hill to put a stone on the cairn.

Knockfierna is one of the most famous fairy hills in Munster and gets its name from the fairy-king Donn Fírinne (Donn of the Truth). Local legend says that if clouds were seen over the hill it meant that Donn and his fairy assistants, were collecting the clouds, and holding them for a short time to warn the people of the approaching rain. As the appearance of mist on that mountain in the morning is considered an infallible sign that the day will be raining, Donn is called Donn Fírinne (Donn of the Truth). The hill is also the setting for 'The Hostel of the Quicken Trees' a tale from the Fenian cycle of Irish mythology. According to the texts, Fionn mac Cumhaill was trapped here by his foster-son Midac's enchantment. A great battle was fought between the Fianna and the allies of Midac before he could be released.