Lios an Tobair, meaning "ringfort of the well", points to a water source that once gave the enclosure its identity, even if no well is obvious to a visitor today. That name has outlasted whatever daily life once played out inside the bank, which is the quiet strangeness of these places: the archaeology survives in outline, but the reason it mattered to the people who built it is preserved only in language.

The site is a univallate rath, meaning it has a single enclosing bank rather than the multiple concentric rings found at more elaborate examples. A rath of this kind is an early medieval farmstead enclosure, typically dating to somewhere between the sixth and twelfth centuries, built to define and protect a family's living space and livestock. At Derra, that bank stands 1.6 metres high on its exterior face, dropping to an average of 0.8 metres on the interior, with a base width ranging from three to five metres. The enclosed area measures roughly 27 metres across on a north-east to south-west axis and 29 metres on the north-west to south-east, making it a moderately sized example. Entry was from the south-east, through a gap of about three metres that still reads as the original entrance. Outside the bank, a fosse, the accompanying ditch that would have reinforced the sense of enclosure, survives as a barely perceptible depression, around four metres wide and just 0.3 metres below the surrounding ground level. Centuries of ploughing and weathering have reduced it almost to nothing, though it has not entirely disappeared.