I passed through the Phoenix Park today, and there were signs up for concert parking in the park itself. If you're coming into the park at Parkgate St (Heuston Station end), continue up the Chesterfield Rd (main rd) until you reach the Mountjoy Roundabout (the one with lots of yellow hi-vis signs - you can't miss it!), and take the first exit (left). Continue straight on, past the Ordnance Survey on your right and the concert parking is advertised on the first left.