At Rathreagh in County Mayo, a late medieval church sits in open pasture on a low drumlin hill, and one of its original windows, a large opening with a slightly rounded head set into the east-northeast wall, was deliberately filled in at some point after the building first went up. The reason becomes clear when you notice the addition tacked onto that same end of the church: a crudely built room, its masonry noticeably rougher than the careful, randomly coursed stonework of the main structure, which required a new doorway of its own and simply made the earlier window redundant.

The church itself is a substantial late medieval structure, measuring roughly 16.65 metres along its length and 7.15 metres across, with walls nearly a metre thick. Both gables still stand to their original height, which is relatively rare for a building of this age left to the elements, and gives the ruin an unusual completeness even in its roofless state. Paired windows, high and wide, are set symmetrically into both the long walls, at the east-northeast end and again at the centre, suggesting an interior that was once well lit. A wide doorway with a slightly rounded head occupies the west-southwest wall. The later addition, measuring about 6.85 metres in length, was built with noticeably thinner walls and has no surviving features beyond a flat-headed doorway nearly 1.9 metres tall. Whatever purpose it served, whoever ordered it built, and whenever exactly that happened, the addition left its mark on the older fabric by sealing off the window behind it.

The church sits on the north side of a subrectangular graveyard, itself occupying the same gentle drumlin rise. The graveyard appears to still be in use or at least maintained, surrounded by agricultural land. The combination of the two structures, the late medieval nave and the anonymous annexe, makes Rathreagh a quietly instructive example of how buildings accumulate change over time, each alteration leaving a legible trace in the stonework for anyone who takes the time to look closely.