Athassel Priory is a superb ruined Augustinian house near Golden, a short trip from Cashel and much quieter than the Rock of Cashel itself. Founded in the late 12th century, it grew into a large monastic settlement, and the surviving walls still give a strong impression of its former scale.
The site is unguided and feels wonderfully unpolished: grass, stone, riverbank, and broken medieval architecture rather than ticket barriers and interpretive polish. Bring sturdy shoes and combine it with Cashel if you want a fuller picture of Tipperary's medieval power.