The word derives from the Irish "cill", meaning a church or monastic cell, a naming pattern that recurs across Ireland wherever early Christian communities put down roots, often in places that have since lost all visible trace of the original settlement. What survives at Killian is the outline of an ecclesiastical enclosure, the boundary, typically a roughly circular or oval earthwork, that once defined the sacred and domestic space of an early medieval religious site.

Ecclesiastical enclosures of this type are among the more quietly significant features of the Irish landscape. They predate the era of stone churches and formal parish organisation, belonging instead to the period between roughly the sixth and twelfth centuries when Irish monasticism shaped both the spiritual and the practical life of rural communities. The enclosing boundary, whether a raised earthen bank, a ditch, or both, marked the distinction between the sacred interior and the secular world beyond it. Within such enclosures there might once have been a small oratory, a burial ground, the cells of monks or anchorites, and sometimes a holy well. Many of these sites were later absorbed into later church establishments or simply forgotten, their earthworks gradually worn down by centuries of agriculture. That the enclosure at Killian remains a recorded monument at all suggests something of its outline has endured in the ground.