Subcircular in form, they average about five metres across, which is modest enough that a person standing at the centre could touch the perimeter wall with an outstretched arm. Some are ringed by shallow ditches, and that combination of low walling and encircling hollow points toward a specific, rather mundane agricultural purpose that nonetheless has its own quiet strangeness.

These structures are thought to have functioned as fionnán enclosures. Fionnán is a coarse grass, sometimes called purple moor-grass, that grows on the wet upland ground typical of south Kerry. Farmers would enclose patches of it to protect the grass from grazing animals, allowing it to grow tall enough to be harvested as a winter fodder or bedding material. It is an unglamorous use of the land, but one that tells you something specific about how people managed life on the Iveragh Peninsula before modern farming methods arrived. The enclosures at Letter were recorded and described by A. O'Sullivan and J. Sheehan in their 1996 archaeological survey of the peninsula, published by Cork University Press, which remains the foundational reference for this kind of field monument in the region.