At the Synod of Rath Breasail in 1111, Glendalough was designated as one of the two dioceses of North Leinster.
The Book of Glendalough was written there about 1131.
St. Laurence O'Toole, born in 1128, became Abbot of Glendalough and was well known for his sanctity and hospitality. Even after his appointment as Archbishop of Dublin in 1162, he returned occasionally to Glendalough, to the solitude of St. Kevin's Bed. He died in Eu, in Normandy in 1180.
In 1214, the dioceses of Glendalough and Dublin were united. From that time onwards, the cultural and ecclesiastical status of Glendalough diminished. The destruction of the settlement by English forces in 1398 left it a ruin but it continued as a church of local importance and a place of pilgrimage.
Glendalough features on the 1598 map "A Modern Depiction of Ireland, One of the British Isles" by Abraham Ortelius as "Glandalag".
Descriptions of Glendalough from the 18th and 19th centuries include references to occasions of "riotous assembly" on the feast of St. Kevin on 3 June.
The present remains in Glendalough tell only a small part of its story. The monastery in its heyday included workshops, areas for manuscript writing and copying, guest houses, an infirmary, farm buildings and dwellings for both the monks and a large lay population. The buildings which survive probably date from between the 10th and 12th centuries.