A second, smaller church stands 120 metres to the south, and the whole complex sits within a cashel, the term for a stone-walled monastic enclosure, of irregular shape. When the Office of Public Works arrived in 1912 to carry out repairs, they found the larger building so engulfed in bramble that its plan could barely be traced from the outside.

The monastery on Inchbofin, whose Irish name Inis Bó Finne means island of the white cow, was founded in the fifth century by Moríog, also known as St. Ríog or Doríog, a nephew of St. Patrick and one of seven sons of Restitutus of the Maca Baird. His feast day fell on the first of August, and a holy well bearing his name, Tobar Ríog, survives about thirteen kilometres to the northwest in Co. Roscommon. In the later medieval period it has been suggested, though without surviving documentary proof, that the Canons Regular of St. Augustine occupied the island and were responsible for expanding the church. The additions attributed to that phase include a transept on the north side and the vaulted sacristy beyond it, with a room above. The last identifiable campaign of building dates to the late thirteenth or early fourteenth century, when two-light tracery windows in the decorated style were inserted into the transept walls. One of those windows, on the north wall, remains complete, its external hood moulding finished with a carved human head. The standout feature, however, is considerably older: a Romanesque window near the north-east angle of the nave, its arch and jambs carved with interlocking chevrons and rows of small pellets, with a fret pattern filling the triangular spaces on the arch ring. Its stones had been displaced by ivy before 1912, when they were carefully reset. Nine early Christian cross-slabs were uncovered during the ground clearance works carried out at the site, hinting at just how much earlier the island's sacred use extends beneath what is visible today.