The Knights Hospitaller, a religious and military order founded to care for sick pilgrims in the Holy Land, established a significant presence here that endured for centuries, yet almost nothing of it remains visible above ground. What does survive has had to be coaxed back into view by excavation, and even then only in fragments.

The priory was founded in 1174 by Richard de Clare, the Anglo-Norman magnate better known as Strongbow, and it answered to the Knights of the Order of St John the Baptist of Jerusalem. Its landholding was considerable, running to some 500 acres in the surrounding area, and it absorbed an older Irish monastery, Cill Maigneann, which had stood on what is now the Royal Hospital site. Historical sources record a layered complex: an inner enclosure containing a castle with a quadrangle and a strong wall punctuated by towers at each of its four corners, and a wider outer enclosure with a gate on the southern side facing the common green at Kilmainham. A parochial church also stood within the precinct, somewhere near the high cross that still stands on the hospital grounds today. In 1859, fragments of decorated floor tiles from that church were uncovered close to the cross. By the time of the Down Survey of 1655 to 1656, the priory was already a ruin, and when the Royal Hospital was constructed in 1681, the old structure described simply as the castle of Kilmainham was demolished and its materials folded into the new building. Excavations carried out by Bersu in 1948 recovered medieval floor tiles, a stone wall, and a well; stonework from the same medieval phase came to light again during restoration work in the 1970s.

The Royal Hospital Kilmainham, now home to the Irish Museum of Modern Art, is open to the public and the grounds are freely accessible. The high cross near which the medieval church once stood is worth locating, since it marks roughly the area where the tile fragments were found in the nineteenth century. There is nothing dramatic to see at ground level, but knowing that the courtyard underfoot was once a fortified Hospitaller enclosure does alter the way the space reads. The building itself was constructed in part from the rubble of what came before, which makes it, in a quiet way, a structure that contains its own predecessor.