The ruins of the Cistercian abbey at Abbeyshrule, known by its Latin name 'Flumen Dei', meaning River of God, contain within a single footprint the overlapping ghosts of a medieval monastic church, a post-Reformation place of worship, and a smaller church built inside that one again. The outermost walls belong to a structure 45.6 metres long; the innermost to a building barely a quarter of that floor area. You can, in effect, stand inside three churches at once.

The abbey was founded in 1200 by Cistercian monks colonising from Mellifont in Co. Louth, under the patronage of the O'Farrells of Annaly, the dominant Gaelic family of the region. Dedicated to the Blessed Virgin, it was affiliated to Bective Abbey in Co. Meath from 1228 onwards. The original church, built in the late twelfth and early thirteenth century, was a long rectangular structure with a crossing tower, a nave, and what appear to have been the beginnings of a south aisle, evidenced by springing stones for two arches still visible in the south wall. In 1475 the abbey was burnt by English forces. After the dissolution of the monasteries in 1536, the chancel, the easternmost section reserved for the clergy, was converted into a working church, with the crossing tower blocked up and three small barrel-vaulted chambers inserted within it. A double bellcote was added on top of the tower's west wall, which then served as the new building's west gable. Queen Elizabeth granted the monastery to Sir Robert Dillon in 1568, and in 1592 it was formally surrendered to the English Crown. By the seventeenth or early eighteenth century, yet another church had been tucked inside the existing one, reusing the north wall and east gable but constructing new walls within, reducing the usable space to 12.9 metres by 7.6 metres. Two lancet windows with roll moulding in the east gable survive from the original thirteenth-century construction and are among the more legible features remaining on the site.

Conservation work carried out in 2010 to stabilise the barrel-vaulted chambers uncovered something unexpected: soil removed from the floor above the chambers contained 5.5 kilograms of disarticulated human bone, the partial remains of several individuals. There was no evidence that these were formal in situ burials, and no clear explanation has been established for their presence. Elsewhere in the graveyard, a carved voussoir, a wedge-shaped stone from an arch, bears the carved head of a bishop or abbot wearing a mitre, possibly removed from a window or doorway of the abbey itself. A high cross that once stood nearby has since been moved to the sacristy of the Roman Catholic church in the village.