I have visited Glendalough (or Glen da lough) about twenty times and still find it amazing that a place so wild can be so close to a capital city. My most recent visit to Glendalough was in November 2005 when I stayed at the Glendalough International Youth Hostel with a total of three other guests. When I left Dublin, it was drizzling and threatening to snow. As I came into Glendalough the sun came out and, despite the cold, conditions were perfect for some very beautiful photographs.
Anyone visiting Dublin should definitely spend a day at Glendalough. Spending a night in the area will also mean that you will avoid the herds of tourists that descend on the place every day during the summer. Ideally, come down during a full moon and hike from Laragh up to the Monastic city along the green trail at night. To wander around the Monastic city by moonlight could be one of the highlights of your trip to Ireland.