There are no buses that go directly from Dublin Airport to Glendalough. So both taking a bus to Wicklow (740) or to Booterstown (700) and from there to Glendalough (183 if you chose Wicklow or 181 if you chose Booterstown) are the possible journeys to get you there. Alternatively, you can head from the airport to St. Stephen's Green, where the bus 181 starts its journey. You can plan your trip, with detailed timetables on Rome2Rio
and the St. Kevin's Bus Services website.
Unfortunately, the tickets for this bus can only be bought on the bus.
I hope you have an amazing trip.