This is a rath, the most common type of early medieval settlement monument in Ireland, consisting of a roughly circular enclosure defined by one or more earthen banks and, typically, an external ditch. The Lassanaroe example is a modest but legible specimen: around 35 metres in diameter, with a bank still standing some two metres high and an outer fosse, or ditch, that runs from south to north and retains a depth of nearly a metre despite centuries of agricultural activity pressing in around it.

Raths were built and occupied primarily between the sixth and twelfth centuries, serving as enclosed farmsteads for individual family groups. The bank and fosse were less about serious military defence and more about marking territory, controlling livestock, and projecting a degree of status in the landscape. What makes the Lassanaroe example quietly interesting is its position: set atop a natural hillock within land that has since been brought into pasture, it would once have commanded a clear view of the surrounding countryside, a practical advantage for anyone keeping watch over animals or monitoring the approaches to their holding. The surrounding farmland has been reclaimed and smoothed over time, which makes the rath's own slight elevation feel more pronounced than it might otherwise, the hillock and the monument reinforcing each other.

The interior is heavily overgrown, which means the earthwork itself is easier to read from the outside than from within. The bank and the line of the fosse are the things to trace on a visit, particularly along the southern and northern ends where the ditch is most traceable.