The cathedral is not alone: it sits within an ecclesiastical enclosure, the kind of curving, often circular boundary that marks out the earliest Christian monastic sites in Ireland. These enclosures, usually defined by an earthen bank or fosse, trace the original sacred boundary of a monastery, sometimes predating the stone buildings at their centre by several centuries.

Clonfert's association with early Christianity is deep. The monastery here was founded by Saint Brendan, known as Brendan the Navigator, traditionally dated to around 558 AD. Brendan is one of the more singular figures in early Irish hagiography, credited in medieval texts with a remarkable ocean voyage, possibly reaching Iceland or beyond. The site he founded grew into a significant ecclesiastical centre, and the cathedral that survives today, with its astonishing carved west doorway, dates largely from the twelfth century, though it incorporates later medieval additions. The enclosure that surrounds it is the physical memory of that original monastic foundation, its boundary shaping the layout of the site long after the community it once enclosed had changed beyond recognition.