The site of a ruined monastery in Ireland, founded in the 6th century by Saint Kevin, who had previously lived there "in the hollow of a tree." Over the centuries the monastery was built up, and, despite the monastery's destruction in 1398 by English forces, several of the buildings remain today. Following are descriptions of what I think are the more interesting ruins at Glendalough, starting with those concentrated in the lower valley, where a visitor center has been built as well.
The gateway now consists of two stone arches, but was once two-storied with a timber roof. It is the only surviving monastic gateway in Ireland.
The Round Tower
The round tower of Glendalough is about 110 feet high and made of mica-slate and granite. It originally had six stories, with floors made of wood and connected by ladders, but the entrance is about twelve feet from the ground for defense, and one can no longer go inside. The four stories above the entrance each had one window for light, and the top story has four windows: one facing each of the cardinal directions. The roof of the tower was rebuilt in 1876 using the original stones. Towers like this one served as bell towers, store houses and places of refuge during attacks.
The cathedral is the largest ruin at Glendalough. It is built of stone and is now missing much of the walls and all of the ceiling, but is still a fascinating structure. The nave and antae were built in the 10th century, and a chancel and sacristy were added on in the late 12th and early 13th centuries. These later parts feature decoration in the Romanesque style. There is a bullaun stone (a pagan tool used for grinding and mixing herbs and medicines) built into an inside wall of the building, but the reason for this is, as far as I can tell, unclear. Some suggest that putting the stone into the wall was an attempt to pull pagans into the Church, while others state that it was placed sideways to prevent its future use. Most guides at the site do not mention the stone (I've watched several), instead focusing on a rather less interesting gravestone planted right next to it.
Saint Kevin's Cross
This is a Celtic cross made of granite and built in the 12th century. It is about ten feet tall and the ring is unpierced. There is a story sometimes told by guides that anyone who can reach their arms around the cross is guaranteed to reach Heaven.
This building is also in the Romanesque style. Its name comes from the practice of burying priests in it in the 18th and 19th centuries, but its original purpose is unknown. This building was at one time much more ruined than it is now, but was partially reconstructed from a 1779 sketch.
Saint Kevin's Church
This church is also know as the "kitchen" because it has a belfry that looks rather like a kitchen chimney. It has an intact roof, built of overlapping stones. The church was built in the 11th century or earlier. This building is locked, and the inside is only accessible if one is on a guided tour, so unfortunately I did not get a good look at it.
There are five other old churches in the valley: Saint Keiran's Church, Saint Mary's or Our Lady's Church, Trinity Church, Saint Saviour's Church, and Reefert Church. There are also a couple other ruins of interest near the upper lake, both of which relate to Saint Kevin:
Saint Kevin's Cell
Saint Kevin's Cell is a beehive-shaped structure built on a rocky spur over the upper lake. It is thought to have been used by Kevin as a home.
Saint Kevin's Bed
Saint Kevin's Bed is a small cave on a ledge above the upper lake. It is difficult to reach, requiring both a boat and climbing. This cave was used by Kevin as a refuge, although it's unclear whether it was a refuge from raiders or his duties as leader of his monastery. There is a story, which I have heard originated just a few hundred years ago, that a woman was attracted to Kevin and eventually came to him naked while he was sleeping in this cave. He, shocked, pushed her off the ledge to her death. "And thus became a saint," one of the guides who told this story added ironically.
Glendalough Visitor's Guide published by Dúchas, The Heritage Service of Ireland
Eyewitness Travel Guides: Ireland published by DK Publishing, Inc.