The building was subsequently enlarged, the new walls rising above that older profile, and the evidence of the change was simply absorbed into the fabric. It is the kind of detail that makes a ruin speak more quietly than any plaque or panel.

The church at Lissagriffin measures roughly eighteen and a half metres in length and just under seven metres wide. It was dedicated to St. Brendan, and its rectory fell under the administration of the College of Youghal, a collegiate church foundation on the other side of the county. The surviving walls carry several points of architectural interest: the south wall is propped on the outside by two buttresses where the central portion of the north wall has already gone, and the doorway in the south wall has an inserted lintel, a later stone head fitted into an existing opening, with a relieving arch built above it to carry the weight of the masonry overhead. The west attic window retains a single ogee-headed light, the ogee being a double curve typical of late medieval Irish ecclesiastical stonework. Just inside the doorway lies a cross-slab, a flat stone incised with a cross, of a type commonly used as grave markers across early medieval Ireland. The east window also has an inserted linteled light, its embrasure splayed outward to draw in more light. A note from 1639 records the church as being in repair; by 1666 it stood vacant, and by 1700 it had already fallen into the condition of ruin that visitors find today.