I only camped once but I think the earlier you get there the shorter the distance to the entrance.
Try and remember you're only going for three nights and not to pack too much as you'll be carrying it to the site. You can pop into Stradbally itself for food supplies over the weekend in the fairly decent Centra/Supervalu, also the food at the festival is really good.
Don't pitch your tent near the toilets as smell and just busyness are not good.
Maybe on your first run from car, bring the tent itself and pitch that, then go back for the rest of the stuff. Also, pick slightly higher ground so that if there is any rain it won't affect you as much.