If you fly into dub and decide to drive down to cork; take an hour or so break in Cashel on the way down and visit the rock of Cashel. It maybe an hour and 40min south of dublin so would be a good break.

Just a side note; When you leave dublin airport to get to the main road to the South (n7/m7) you have to go around the main Dublin ring road. Part of this is a toll road (without a payment booth). If you ask at the car company when you collect your car im sure they will explain how to pay.