Lios Dún na bhFlann, roughly translated as the ringfort of the fortification of Na Flainn, carries within it a plural personal name, Flann, suggesting that whatever family or group once held this ground, they were significant enough to be remembered in the landscape long after they had gone. A ringfort, to give a quick definition, is a roughly circular enclosure bounded by one or more earthen banks and ditches, used throughout early medieval Ireland primarily as a farmstead or settlement. This particular example sits on boggy ground to the south-east of the Tonalassa promontory fort, on the north Kerry coast, placing it in a cluster of ancient enclosures that once made this stretch of coastline considerably more inhabited than it appears today.

The structure itself is modest but legible. The raised inner area measures roughly 11.5 metres by 10 metres, enclosed by a shallow fosse, which is the ditch running around the interior platform, and an outer earthen bank. The fosse is 1.8 metres wide and sits about 0.3 metres below the level of the raised inner ground; the bank rises around a metre above the fosse and another metre on its outer face, with a base width averaging 3 metres. None of this is dramatic in scale, but the proportions are consistent with a domestic enclosure rather than a defensive one. To the south there is a 4-metre gap in the bank, and the entrance proper lies to the east, where a single slab measuring 1 metre by 0.5 metres survives as the southern jamb stone of what would once have been a more defined doorway. These details were recorded and published by C. Toal in the North Kerry Archaeological Survey of 1995, which remains the primary source for understanding this site and others like it across the region.