Hare Island, known in Irish as Inis Aingin, sits at the south-eastern end of the lough, and the ground here still holds the shape of an early medieval ecclesiastical enclosure, its curves legible in the fields surrounding a Romanesque church. What makes this place quietly remarkable is the sequence it implies: a saint who began here, then moved on, leaving behind a community, a landscape still marked by its original boundaries, and the dead buried within those boundaries for generations.

According to O'Hanlon, writing in 1873, St. Ciarán established a monastic foundation on Inis Aingin in the early sixth century before departing for Clonmacnoise, where he would found one of Ireland's most significant early Christian sites. He was succeeded here by Enna Mac Hui-Laigsi as abbot. The enclosure that survives, recorded in detail in 1978, is oval in plan and bivallate, meaning it was defined by two concentric boundaries rather than one. The inner enclosure measures roughly 53 metres north to south and 47 metres east to west, defined by a broad earthen bank; a second, lower bank extends outward in an arc from the old shoreline on the west, curving north and back down to the shoreline on the east. Between the two banks, several radial banks cross the space, and two roughly parallel ones on the western side may mark an original entrance. To the north, a large subrectangular area enclosed by a further low bank may represent an annexe to the monastic complex, a feature found at other early Irish monastic sites where additional functions, storage, guest accommodation, or farm use, required their own defined space. Local reports of human burials discovered both inside the inner enclosure and in the zone between the banks suggest the whole area served at various times as a graveyard. In 1822, George Petrie recorded a ninth-century cross-slab lying on the ground inside the enclosure, a carved stone that had already, by that point, been lying there long enough to seem simply part of the place.