The RSPB have recently put up signs saying "No overnight parking. Car park will be locked between 20:00 and 08:00". I left last night at 8 and there was no sign of anyone locking up, but there was a very shiny and weighty chain and padlock there. So watch out if you're planning a trip, parking may not be possible there between 20:00 and 08:00!
There is a layby a few hundred metres up the road towards Croachy that can be used for parking if needs be (and should be used if the parking is full). I think there may also be parking up towards the houses at the end of Loch Ruthven, but I'm not sure.