What marks it out is subtler: a band of differential vegetation, roughly four to five metres wide, traces the curving arc from south-west to north-east, and at the time it was recorded this vegetation took the form of a dense growth of tall ferns. The straight side of the D requires no earthwork at all; the land simply drops away sharply to a vertical scarp at the shoreline, the sea doing the work that elsewhere a rampart might.

Enclosures of this kind are among the more ambiguous categories of Irish field monument. Without excavation, their date and function remain open questions; they could be early medieval, prehistoric, or later still. What gives this one additional context is the presence of a mound barrow lying only about ten metres to the west. A barrow is a burial mound, typically prehistoric in origin, and the proximity of the two features suggests that the southern end of Bartragh Island carried some significance over a long stretch of time, possibly accumulating monuments the way certain elevated or liminal spots tend to. The enclosure itself measures roughly eighty to eighty-five metres on its longer east-north-east to west-south-west axis, and forty to forty-five metres across, with what appears to be a broad entrance gap of ten to fifteen metres on the northern side.

Bartragh is a tidal island in Killala Bay, which shapes any visit considerably. The enclosure sits in pasture, and the fern bank that once marked its boundary may have shifted or thinned depending on the season and the grazing pressure. The entrance gap to the north is the feature most likely to read clearly on the ground, while the southern boundary, where the scarp falls to the shore, gives the whole site its most legible edge.