This moated site forms a rectangular enclosure measuring 40 metres long by 28 metres wide, oriented northwest to southeast. The structure consists of two concentric banks with a defensive ditch, or fosse, running between them. A narrow causeway, just 3.5 metres wide, provides the only access through these earthworks along the eastern side.

The outer bank, though weathered by centuries, remains most impressive along its northern edge where it rises to 1.2 metres high with a flat top spanning 2.4 metres. Between the banks, the fosse varies in character; it's broadest in the southern section at over 5 metres wide, whilst reaching its greatest depth of 0.85 metres on the western side. The inner bank features raised corners, except in the northeast where later activity has partially removed it, and stands best preserved on the western side where it measures over 10 metres wide at its base. Recent geophysical surveys have revealed that this moated site was carefully positioned to respect an earlier double circular enclosure associated with the nearby church, suggesting a continuity of significant occupation at this location.

The earthwork likely dates to sometime after the twelfth century, when Lambay Island was granted to the Archbishop of Dublin following the Anglo-Norman invasion of Ireland. This rectangular fortification would have served as an administrative or defensive centre for the church's holdings on the island, its substantial banks and deep fosse providing both protection and a statement of authority. Though the interior has since been cultivated and the southeastern sections have suffered damage over time, the site remains an evocative reminder of medieval power and land management on this small island off Dublin's coast.