In the latter part of the sixth century, St. Kevin crossed the mountains from Hollywood to Glendalough. The path he took later became known as St. Kevin's Way. This track facilitated the development of Glendalough, so that within 100 years it had developed from a remote hermitage site into one of the most important monastic sites in Ireland.
It is not known where the original settlement in the Glendalough Valley was located but was possibly in the same area as Reefert Church, on the south-eastern shore of the Upper Lake. Reefert was a sacred location as it was the burial place of kings and chiefs. Later still, the cathedral, churches and the round tower was constructed to the east of the lower lake. This gradual eastern progression of the settlement made it increasingly more accessible to would-be pilgrims and travellers.
As its fame spread, the monastery flourished and its success continued after St. Kevin died in 617AD. By the end of the eighth century an impressive monastic settlement, capable of supporting a sizeable population, had been acquired. Monasteries in pre-Norman Ireland were a considerable economic force, and were sufficiently well organised as to be capable of withstanding periodic crises and famines. There was a large lay population in Glendalough. The population there may have been around 500-1,000 people. Many of the laypersons would have been employed by the monastery - tending flocks, tilling, sowing and harvesting. In addition to stores of treasure, most monasteries maintained substantial stocks of food.
In the aftermath of famines, disease, drought and other natural disasters monasteries were often plundered. The location of the Glendalough monastery, in a remote valley in an upland area, made it an easy target, and it was attacked many times between 775 and 1095 by both local tribes and Norse invaders. Usually the churches and houses were burned, but each time the monastery was rebuilt. That the monastery survived this constant onslaught is remarkable and is evidence of the large number of people living there as many would have been needed to rebuild after each attack.
With the arrival of the Anglo-Normans a dramatic change occured within the political landscape of Ireland. Both ecclesiastical and political activity centred around Dublin. Subsequently, Glendalough was annexed to the diocese of Dublin and its importance declined. Despite this, the place has retained a spiritual significance.