Ringforts, known in Irish as raths, were the enclosed farmsteads of early medieval Ireland, typically circular or oval enclosures defined by earthen banks and ditches, and they almost always have a single original entrance, usually facing east or south-east. At Tullassa, that entrance has simply vanished. The site is defined instead by a continuous scarp, a low step in the ground rather than a built-up bank, rising from around 0.8 metres at the south-west to 1.4 metres at the north-east. Two modern gaps have been cut through the perimeter at the north-north-east and south-west, presumably to allow agricultural access, but neither corresponds to any original opening that surveyors could identify.

The enclosure is a reasonable size, stretching roughly 61 metres east to west and nearly 49 metres north to south, sitting at the eastern end of a gentle ridge running roughly east-north-east to west-south-west. Whether the original entrance was obliterated by ploughing, eroded away, or was simply always less obvious than at comparable sites is not recorded. For much of the late twentieth century the site was catalogued only as an "Enclosure" rather than a ringfort proper, appearing under that cautious designation in the Sites and Monuments Record of 1992 and the Record of Monuments and Places of 1996, a reminder that official classification sometimes lags behind what a site actually suggests on the ground.