Alongside them, the martyrdom of St Sebastian and a depiction of the Holy Trinity survive on the same plastered surface, dating to the fifteenth century and constituting one of the more remarkable concentrations of medieval wall painting left in Ireland. The fact that they endure at all, inside what is otherwise an open ruin exposed to the west of Ireland sky, makes Abbeyknockmoy worth pausing over.

The monastery was founded in 1189 to 1190 by Cathal Crobderg O'Conor, King of Connacht, as a Cistercian house, an order known across medieval Europe for its austerity and its preference for remote, well-watered sites. The church, which runs east to west and extends to some 60 metres in length, belongs to what architects call the Transitional style, that early thirteenth-century moment when Romanesque rounded forms were giving way to the pointed arches of Gothic. The building has an aisled nave, a chancel with a ribbed vault and east-facing windows, and two transepts each containing a pair of barrel-vaulted chapels. Three of the four crossing arches are now walled up, work believed to be broadly contemporary with the insertion of a central tower, probably in the fifteenth century, of which only a ruined shell remains. To the south, the claustral buildings and cloister survive in varying states of decay; the east wing, containing the sacristy, chapter house, and a garderobe (a medieval latrine built into the wall), is the best preserved section. A rectangular building of likely post-medieval date sits immediately to the north of the church, and a field system and three holy wells are also associated with the complex. About 325 metres to the west, a modern mill is said to stand on the site of the original abbey mill.

The paintings in the chancel are protected from direct weather by the surviving vault above them, which accounts for their relative legibility. They reward a close look, particularly the Three Dead and Three Live Kings sequence, a moralising allegory that circulated widely in late medieval Europe but rarely survives in Irish contexts with this degree of completeness.