The enclosure measures roughly 28 metres east to west and 26 metres north to south, its boundary formed by a shallow fosse, a type of defensive or demarcating ditch, roughly 0.8 metres deep, with an earthen bank rising about 1.5 metres on both its inner and outer faces. The interior is grass-covered and tilts gently down towards the south-west, giving the space an oddly bowl-like quality when viewed from within. A slight depression on the eastern side may be all that remains of an original entrance.

This kind of earthen enclosure is a familiar, if still poorly understood, feature of the Irish countryside. Ringforts, or raths, were typically used as enclosed farmsteads during the early medieval period, roughly between the fifth and twelfth centuries, though some enclosures of similar form may be prehistoric in origin. The bank and fosse arrangement at Ballygrillihan follows that general tradition, though no specific date or associated finds are recorded for this particular site. The external bank is broken at both the south and north, whether through deliberate later removal, agricultural pressure, or simple decay is unclear. The fosse itself has become overgrown with briars, which is common on sites that have been left undisturbed at their margins while the surrounding land continues to be farmed.