Glendalough means the valley of the two lakes and it‘s located in county Wicklow. It‘s an old monastic settlement in a spectacular location. There are daily bus tours there but I would recommend doing it on your own. Every day, from the bank past Stephen‘s Green Shopping Centre, at about 11 am (but pls check the schedules), Kevin Buses drive you there, passing Bray and the Wicklow Mountains. The trip takes about 2 and a half hours, so maybe it would be wise to have your lunch on the bus.
When you arrive at Glendalough you‘ll have about 3 hours to wander around and explore the settlement. As soon as you arrive and go through the gate you‘ll see a fine round tower and a cemetery with two beautifully decorated Celtic crosses. Just below it you‘ll see the oldest building: an oratory or little church called Saint Kevin‘s Kitchen. Continue along the path until it forks: go fist left and see some ruins of nice old churches, then go back and follow the path though the wood and along the lake: here again you‘ll come across other interesting monastic ruins.
If you continue straight on you‘ll come to the end of the lake: stop on the beach and look around; this is where they filmed the famous scene of the hand stretching out of the water in the movie "the lady of the lake". If you continue along the path you‘ll walk around the lake and eventually end up at the entrance gate again. If you have any spare time (quite unlikely) there‘s a cosy pub where you can have a drink before catching the bus back to Dublin
If you have time, however, I would strongly suggest you bring a tent and stay overnight. From the pub, in less than an hour walk, you can follow a path in the forest which leads you past the two lakes to the end of the valley. There you can find the ruins of an old tiny mining town where you can set up camp, and the following morning you can walk up the ridge of the mountains and get compelling views over the two lakes and the surrounding area.