Beneath the earthwork, excavators found evidence of an earlier organised landscape, one that the fort's own construction had partly buried and partly consumed. A linear trench, running on a northeast to southwest axis, crossed the interior of the ringfort off-centre, and turned out to predate the whole structure. A ringfort, for context, is a roughly circular enclosure defined by one or more earthen banks and ditches, typically dating from the early medieval period and associated with farming settlements. The trench at Lisduggan was not a feature of the fort at all; it belonged to whatever came before it.

Excavation by Twohig in 1990 traced the trench for forty metres and confirmed that it continued beyond the ringfort in both directions. At its base it measured 0.58 metres wide, opening to 1.3 metres at the top, and reached a depth of 0.9 metres. A cutting through the bank in the southeast quadrant of the fort revealed something particularly telling: the soil dug from that trench had been used to build a small field bank running alongside it. That bank had survived to a height of roughly 0.3 metres at the point when the ringfort's own bank was constructed directly over and around it, partly incorporating the earlier material. The trench itself had been only partly backfilled before the fort was built, meaning that whoever raised the ringfort was working around, and effectively erasing, a pre-existing field boundary. Further similar trenches were identified approximately 350 metres to the northeast, associated with two additional ringforts in the same area, which suggests this was not an isolated arrangement but part of a broader pattern of earlier land organisation across the townland.