Is it easy to get it across on the ferry and into the festival, or is it easy enough to buy it there?
No idea, going myself and assuming the safe bet is to wait till ur in and get it rather than risk bringing it in. That said if ur in ur own car should be pretty easy to smuggle
I've been a few times, they sometimes have dogs at the gates and a few people searching. Best bet is to take an amount you can afford to lose, assess the situation when you get there and throw it away in one of the amnesty bins if it looks like too much. I've gone through just by stuffing it in my boxers before, but I've also thrown away stuff when I've seen dogs.