But behind all the hype, the Chili Peppers are more New Age cool than old-fashioned, hell-raisin' rockers.
They've requested a meditation room, where they will listen to waterfall sounds and imbibe the soothing effects of candles and incense.
Apparently, they want the reflective hideaway as part of their pre-concert chill-out routine before hitting the stage and going "mad".
Finishing touches were being put to the headline act's special backstage retreat yesterday.
The Chilis' pre-concert diet will consist of rice cakes, carob-coated peanuts, pitted dates, cashew nuts and ginger root as healthy a combination as one can find.
If you're not a fan then steer clear of the Whitefields concert site which is at the Castleknock end of the park. There is no parking on all approaches to the Phoenix Park.
Billed as the biggest gig of the summer, the Californians will play to 100,000 fans. Support bands include Irish bands Hothouse Flowers and The Thrills, live dance act Groove Armada and reformed legends The Pixies. The gig gets under way at 2pm.
Since 11pm last night all gates into the venue have been closed to traffic and the park will not reopen until tomorrow morning.