Found this link to a blog over on Outdoors Magic and thought it might be useful.
http://www.mourne-mountains.com/search/label/Campsites
I spoke to the chap that runs this blog a few weeks back about parking in the Mournes and invited him to join.
It's an excellent blog and well worth a read.
I know half the fun is finding your own spots but it is a handy resourse.