Cartographers recorded it clearly enough on the 1838 Ordnance Survey six-inch map, a D-shaped enclosure drawn in solid line on a low ridge above waterlogged ground, yet it vanished from all subsequent map editions. Locally, however, it never disappeared at all. It is still known as Rath Eilbhin, sometimes rendered as Rath Elvin, a name that carries the kind of specificity suggesting it was never truly forgotten, just overlooked by the surveyors who came after.

A rath is an early medieval enclosed farmstead, typically circular, defined by one or more earthen banks and ditches and used as a defended homestead and livestock enclosure. What survives at Carrowcastle is most likely only a fragment of such a structure. The remains form a raised earthen platform roughly fifteen metres across in both directions, irregular and D-shaped in plan, defined on the east and south by a curving scarp rising to about 1.8 metres at its highest point. The western edge is marked by a straight scarp around 1.1 metres high, and to the north a quarry pit approximately twelve metres long has cut into the feature. Taken together, the surviving earthworks suggest this is the south-eastern quadrant of what was once a complete circular enclosure. The rath sits on slightly elevated ground overlooking flat, wet terrain to the south and south-east, a position typical of early settlement sites where a commanding view and dry footing both mattered. To the west, rising ground carries the remains of a church and an earthwork castle, suggesting this corner of Mayo was once considerably busier than it appears today.