This romantic ruin is undoubtedly the most striking of Ireland's historic sites, set as it is on a rocky escarpment high above the Tipperary plain. St. Declan established a church here in the sixth century. The Rock became a stronghold of the high king Brian Boru (who died in battle with the Danes near Dublin in 1014). Cashel was the site of two twelfth-century synods (church councils) by which the ancient and autonomous Celtic church of Ireland was brought into line with the wider Roman reforms (dioceses, clerical celibacy) and also brought under the jurisdiction of the Catholic Anglo-Normans.

The Round Tower is a fine example of 11C ecclesiastical architecture, but the greatest masterpiece of the period is the Romanesque Cormac's Chapel, built in 1127 by Cormac McCarthy, King of Desmond. The much larger Norman cathedral which now dominates the site was built in 1235 (replacing an earlier one built by the King of Thomond in 1169, who gave homage here to King Henry II of England in 1171). The now roofless cathedral has tombs from the sixteenth century and sculptures representing the Beasts of the Apocalypse (Book of Revelation). Edward Bruce held a parliament here in 1315. Gerald, Earl of Kildare set fire to the cathedral in 1495. When charged with the crime by King Henry VIII, he gave the acceptable excuse that he had thought the archbishop was inside! The Castle of the Archbishops at the western end of the cathedral was added by Archbishop Richard O'Median (1406-1440).

Hidden behind the Rock in the valley is another fine ruin, that of Hore Abbey, a Cistercian site built in 1272.