A flattened earthen bank, barely knee-height in places and topped with fallen concrete posts, is not the most dramatic boundary marker in the Irish countryside. But the D-shaped enclosure it traces at Castledillon in County Kildare encloses a site whose origins stretch back to a Celtic hermit saint, a medieval church, multiple burial grounds, and a fine carved stone effigy that has since been removed from the spot where it once stood.

The place was not always called Castledillon. Early scholarship recorded its older name as Diseart Iollathan, meaning St. Illan's Hermitage, a diseart being a type of early monastic retreat or hermitage, typically associated with a single ascetic figure rather than a large community. The name was eventually corrupted to something like Tristledelan before the site acquired its current placename entirely. St. Illan himself was venerated on the 2nd of February, though little else of his story has survived into the documentary record. The enclosure, measuring roughly 52 metres east to west with a notably straight western side running some 61 metres, is characteristic of early ecclesiastical foundations in Ireland, where a roughly circular or oval boundary, known as a cashel in stone or a rath-like bank in earth, defined the sacred precinct. Within this one, a medieval church was later built, and the ground contains a graveyard alongside what may be a separate children's burial ground, a cillín, which in Irish tradition was often reserved for unbaptised infants and sited slightly apart from the main Christian cemetery. The most striking object once kept here was the Castledillon Stone, a 13th-century carved effigy that has since been moved from its original location within the enclosure.

The enclosing bank, about six metres wide at its base, is low and in places heavily disturbed, with a shallow outer fosse, or ditch, still visible around it. Several gaps now interrupt the bank, though the original entrance is thought to have been somewhere along the northern side.