In reply to skarabrae:
I've never taken a car to Arran.
I have found the busses connect well with the ferries.
And if it is Goatfell you want, that is an easy walk from the harbour, though you can get the bus to a number of other start points.
There are several campsites on the island, all of which can be reached by a bus that meets the ferry.
http://www.ukcampsite.co.uk/sites/townlisting.asp?town=Isle%20Of%20Arran&am...
Of course, this being the People's Republic of Scotland you can wild camp anywhere that is wild (ie not ploughed fields or people's gardens).