What survives of the old church at Rathernan in County Kildare amounts to little more than a smothered fragment of wall rising from an unexpectedly prominent mound in the southern part of a graveyard. The remains, mostly the southern portion of the west gable, stand roughly 3.8 metres high and run about five metres in length, with the wall itself nearly three-quarters of a metre thick. By 1986 the masonry had been identified as roughly coursed mortared sandstone blocks, but the stonework is now entirely obscured by ivy, so the structure reads less as a ruin than as a shaped mass of vegetation. A two-metre gap in the gable marks where a window was at some point robbed out, its dressed stone presumably taken for use elsewhere, a common enough fate for medieval building material in rural Ireland.

The mound on which the church sits is circular, around 22 metres in diameter and rising between 1.2 and 1.8 metres above the surrounding ground. That kind of deliberately raised, circular platform beneath a medieval church often signals an earlier, pre-Christian significance to the site, though the record here does not say so directly. What the historical sources do note is that Saint Peter's Day was formerly kept as the patron day of the parish, a tradition of local pattern days, communal gatherings held at holy sites on a patron saint's feast, that largely faded across Ireland during the eighteenth and nineteenth centuries. The ecclesiastical connection to the wider area runs deeper still: the rectory of Ratherne, as the place was then called, was among those granted to Sir Edmund Butler by an inquisition dated 20 May in the tenth year of Elizabeth I, that is 1568, a detail recorded by the antiquarian Mervyn Archdall in his Monasticon Hibernicum under the entry for Great Connell, the Augustinian priory a few kilometres to the south.