These grounds, known in Irish as cillíní (singular: cillín), were informal burial places reserved for those who could not, under Catholic Church rules, be interred in consecrated ground. Unbaptised infants made up the largest proportion of those buried here, though the category sometimes extended to stillborn children, women who died in childbirth, and others considered outside the boundaries of the Church's grace. The result was a scattered geography of small, unmarked, and largely unrecorded graves, occupying old ringfort interiors, coastal margins, townland boundaries, and other liminal spaces.

The use of such sites was shaped by a theological position, rooted in the doctrine of limbo, which held that unbaptised souls could not enter heaven. Families, unwilling to bury their children entirely without ceremony or place, found their own solutions in these informal grounds, which were often associated with pre-Christian or early Christian sites. In this way the cillín at Derreen belongs to a long continuum of folk practice that persisted well into the twentieth century, in some areas until the 1960s and beyond. Clare, with its dense concentration of early medieval remains and its strong local traditions around landscape and memory, has a significant number of such sites, most of them modest in scale and easy to overlook.

Because the historical record for this particular site at Derreen is currently sparse, the finer details of its use, including when burials took place and over what period, remain to be established. What can be said is that cillíní in general are fragile survivals, their boundaries often unmarked by anything more substantial than local memory or a slight rise in the ground. Visitors to such places should move carefully and treat the ground with corresponding respect.