For what it's worth, I took a walk around the exterior of Croke Park today as it is very near my home.
First, I agree with mk310174 on this one and the U2 gig will be no exception.
There are five turnstiles each at section A and section K. The turnstiles in section K lead on to the street and the turnstiles in section A are inside the entrance gate on the Foster Avenue side. Technically entrance K probably gives people an advantage but one of the main concerns for security on the day of the gig will be to ensure that concert goers are not blocking up the streets leading into the venue, so the entrance gate at section A will probably be opened well before the gig starts and punters will be held in a 'corral' before being allowed through to the turnstiles.
The area inside the entrance gate on Foster Aveue is quite large so they may allow the 'early birds' at section K into that area as well. There is immediate access to that area from the Clonliffe Road side as well, where section K is located.
Croke Park is in a busy residential area with private housing on all sides. Promoters will be anxious to ensure that disruption to local residents is kept to a minimum. They want to continue running concerts at the venue.