What marks it out is the subtle geometry still readable in the landscape: a slightly raised, subcircular platform, roughly 34 metres from north-north-east to south-south-west and 33.5 metres east to west, enclosed by an earthen bank standing about one and a half metres high. That bank has held its shape for over a thousand years, and the break in the slope on which it sits seems almost chosen, giving the enclosure a natural prominence that is understated rather than dramatic.

This is a rath, the Irish term for a ringfort built from earth rather than stone. Ringforts were the dominant form of rural settlement in early medieval Ireland, broadly from the fifth to the twelfth centuries, and many thousands survive across the country in varying states of preservation. Most were farmsteads, enclosing a homestead and its immediate activity areas within a bank and external ditch, offering a degree of protection for people and livestock. The earthen bank at Brittas, still at a height of 1.5 metres, is a reasonable survivor given that agricultural land has claimed the ditches and interiors of countless comparable sites. The subcircular shape is typical; true circles are relatively rare, and slight irregularities in plan often reflect the terrain or the practical limits of early construction.