Apparantly that option took hours on Sunday, but might be the best of a bad bunch and there are buses too.
Getting back on the train can take a while I hear.
For U2, we parked on rockfield road, dropped a car early in the day before parking restrictions were on. You have to check though - sometimes there are restrictions on that road (or others). It's a 10 minute walk to the stadium, but compared to a long wait for trains, is probably quite a good alternative.
Finally, the other option is to pay for parking somewhere nearby.
Getting away is always the headache, good luck