This was not a boundary marker or a drainage feature. It was, in all likelihood, a frontier fortification, one designed to control who crossed the river and who did not.

The earthwork belongs to a family of monuments associated with the Black Pig's Dyke and the Danes Cast, the collective name for a loose chain of linear earthworks that once demarcated the edges of Ulster. At Cloonavery, the main northern line is a substantial piece of engineering: an inner bank roughly 30 metres wide at its base and up to 6 metres high, separated from an outer double bank by what early observers described as a paved causeway, between 4 and 10 metres wide. Two entrances, each between 16 and 23 metres across and fitted with inturned horn-shaped extensions of about 40 metres, were positioned roughly 80 metres and 650 metres from the eastern end; a geophysical survey in 1990 suggested that the eastern entrance may have held a central gatepost. A second, lower line of defence survives about 60 to 100 metres to the south, though only its eastern section remains, running for around 300 metres. Researchers Tom Condit and Victor Buckley interpreted the whole system in 1989 as a Connacht defensive line controlling fords both within the Shannon loop and to its west, in the area known as Cuiltyconway. Excavation in 1991 supported this reading and added a striking detail: a calibrated radiocarbon date of 338 to 44 BC, placing the construction firmly in the Iron Age, though the evidence suggests the main rampart was later enlarged, implying more than one phase of use or concern. At its western end the earthwork meets the Shannon itself, and roughly 370 metres to the southwest a further section of double banks continues parallel to the river, extending the system still further.