The Office of Public Works visitor centre at Glendalough is the sensible starting point for a visit — it has a small museum covering the history of the monastic settlement from St Kevin's founding in the 6th century through the medieval period, with original stonework and reconstructed artifacts. The introductory film is worth watching before you walk the site.
The centre also sells the walking maps you need for the longer routes in the national park. Admission to the visitor centre is charged; the monastic site itself is free to walk around. The centre's car park fills early on summer weekends and bank holidays — aim to arrive before 9am if you can.