Somewhere beneath the landscaped grounds of a former demesne on the eastern bank of the River Barrow, two religious foundations have effectively vanished from view. Not one stone of the early monastery, nor one visible wall of the Cistercian abbey that followed it, can be seen above the surface today, yet the ground itself keeps yielding evidence that both were very much here.

The story begins with St. Evin, who according to scholarly tradition founded a monastery at a place called Rosglas na Muimneach, likely in the early medieval period. A branch of that monastery is said to have maintained St. Evin's bell as a swearing relic, a practice by which an oath was bound with particular gravity to a saint's sacred object, at what is now a yew tree cemetery north of the town of Monasterevin. The early monastery was followed, probably on the same site, by a more substantial foundation: sometime between 1177 and 1181, Dermot O'Dempsey, King of Offaly, founded an abbey that shortly afterwards was absorbed into the Cistercian order. Known as the Abbey of Rosglas, or in Latin 'de Rosea Vallis' (meaning 'of the Rose Valley'), it was dedicated to Saints Mary and Benedict. The Cistercians were a reforming monastic order that favoured remote, well-watered sites, and the River Barrow plainly suited that preference. The abbey itself has left no standing fabric, though its foundations may be folded into the walls of the later Moore Abbey house on the same ground. A medieval font now sitting in a modern chapel nearby may have originated in the abbey church. A survey map drawn in 1759 by a cartographer named Scale shows a church to the north of the house; by then it was already a ruin, and today nothing of it remains.

The site has not given up its secrets quietly. Burials were uncovered just east of the house during the mid and late nineteenth century, likely connected to the monastic community. Then in 1996, archaeological monitoring of service trenches revealed further burials to the north of the house, along with ironworking debris, stray fragments of medieval pottery, and a timber-lined pit of possible medieval date situated on what was once the original bank of the Barrow. Whether that pit served as a sluice, a drain, or some kind of dock is still uncertain, but its position on the old riverbank hints at a community that made practical, daily use of the water running past its walls.