Tigernach of Clones lived in the late fifth and early sixth century. He was reputedly a grandson of an Ulster king and received his religious education in part at Whithorn in Galloway. According to medieval tradition he received the benediction of St Maccartin, was granted the bishopric of Clogher, and performed miracles widely reported in Ireland, Britain, and France. Folklore says an angel directed him to build his monastery in the territory of his grandfather. He dedicated the foundation to St Peter and St Paul. The cult of Tighernach, the version of his name in Old Irish, was promoted by a branch of the Ui Cremthainn who migrated into the Clones area in the eighth century and made his veneration into a regional industry. Even into the sixteenth century, the position of coarb of Clones, the hereditary lay successor to the saint, was passed down through families like the McMahons.

Clones Abbey was destroyed by fire in 836, again in 1095, and again in 1184. Each time the monastic community rebuilt. In 1207 Hugh de Lacy, Earl of Ulster, razed both the abbey and the town. Five years later, in 1212, the English rebuilt them and added a castle. Around this period the foundation appears to have been formally reconstituted as an Augustinian house, and the Romanesque church whose ruins stand on McCurtain's Street today probably belongs to this twelfth-century reconstruction. Look closely at the lower courses of the round tower on the side opposite the door. The masonry there is cracked in a way that suggests heat damage, almost certainly a souvenir of one of the medieval burnings.

In the centre of Clones, on the Diamond, the medieval marketplace, stands a fifteen-foot sandstone high cross. The shaft and head are separate pieces, the shaft older and dating from roughly 825 to 875 AD. Interlacing beads run up its sides and form a collar at the top, in the high-classical style of Irish sculpture. The faces are covered in narrative panels: the Fall of Man, the Adoration of the Magi, Daniel in the lion's den, Abraham's sacrifice of Isaac, Adam and the tree and the serpent. Turn the cross around and the New Testament takes over: the multiplication of the loaves and fishes, the miracle at Cana, the baptism of Christ. Each panel is a single comic-book frame in stone, dramatic and gestural, and the whole monument is essentially a stone Bible for a population that could not read.

In the abbey graveyard sits a stone sarcophagus shaped like a small steep-roofed house, decorated with worn animal-head carvings, traditionally said to contain the bones of St Tigernach. It is what archaeologists call a skeuomorph, a stone copy in miniature of an earlier wooden shrine, of the kind once used for relics of saints in early Christian Ireland. The sarcophagus probably dates from the late twelfth century, replacing or imitating a wooden original. The animal-head carvings are too eroded now to read in detail, but the architectural form, the steep house-shaped roof, the corner finials, the rectangular body, is unmistakable. The relics inside, if any actually remain there, were placed in such shrines so pilgrims could touch the lid and hope for healing.

The four pieces of the medieval monastic complex are scattered across the modern town. The round tower is in the old graveyard off Abbey Street, where the small Romanesque arch and window of the twelfth-century church also stand. The sarcophagus is nearby. The high cross is on the Diamond, a short walk away. A drawing from around 1587 shows the church of the Augustinian abbey, labelled simply a churche, still standing at the time with its tower and choir intact, east of the round tower and west of the surviving small Romanesque ruin. The Reformation under Henry VIII suppressed the monastery in the 1530s. By the seventeenth century the abbey was in ruins, although solitary monks lived in the neighbourhood until well into the eighteenth century. An English garrison was billeted in the ruins for a time. The Augustinian community is long gone. Their building stones are not.