The west gable is entirely gone, the choir arch has fallen, and yet the east window still stands, its semi-circular head formed from a single cut stone, a detail that speaks to careful, considered construction at some point in the medieval period. The walls of the nave and chancel, built from large limestone blocks laid irregularly and mortared with lime and sand, still reach roughly fourteen feet in height in places, giving a strong sense of the building's original scale despite what has been lost.

The place-name shifts recorded by the antiquarian Thomas Johnson Westropp trace the site across several centuries: Rathnaseir appears in documents from 1201 to 1204, and around 1220 a Roger Waspayl granted lands nearby to one Matthew, parson of Rathnesser, a transaction preserved in the Black Book of Limerick. By 1336 the name appears as Rathnaser in termino Cragg, and later rental records from 1418 mention an ecclesia and chapel. The church itself is built of well-dressed gritstone, a harder material than the limestone of the walls proper, and the chiselled brown stone used for the window surrounds and quoin stones, the dressed corner blocks that give a structure its edges and strength, is notably distinct from the rubble limestone of the side walls. By 1840, when an Ordnance Survey correspondent described the building in careful detail, the choir arch on the middle gable had already been totally destroyed.

The site sits within a small graveyard that was, at the time of the 1840 survey, still actively in use, which means the ground around the ruins is likely to have remained at least partially maintained over the years. The east window is the feature most worth examining closely; from the interior, its splay arch measures roughly 2.2 metres by 1.3 metres, narrowing considerably on the outside face, a typical arrangement for admitting light while limiting draught. The south wall of the nave also retains traces of two windows, one now largely defaced, the other preserving a round arch of the same brown stone on the interior side, though little of the dressed stonework survives on the outer face.