As Anthonyyy has said it is not legal to camp in the Glen but as the Wicklow way runs through it and if you are discreet(sp) camping out side of the Glen, it just of the trail is tolerated. The rangers are OK if you resepct them and abid by what they say(if you are total to move then do so it's usually for a reason many areas are under special pretection but are not always mark) they will not bother you further if you are as I said discreet this is my experience only do.
Hostelling the wicklow way is expensive so camping has to be expected. There are some nice official campsites in Roundwood and surrounding areas but they are expensive at high season too (30euro a night no fires but showers and it is of the trail). Also this time of year is best as the weather keeps the far weather ramblers away poking their noise into your tarp.
I should post this in the meetings but if anyone is free next month give me shout more then willing to meet up for even a day hike (once you don't mind someone that like to move at a slow pace)
James