We didn't have an international driver's license. No problem. You don't need it.
Don't sweat too much about the driving. As stated before, Italians are good drivers, and have surprisingly few accidents. What you need to watch out for (and we had it be the person in the passenger seat) is when you're in/near a town WATCH OUT for the motorcycles & vespas. Often they are kids riding double (or in one case in Sicily, 2 kids on a vespa with one holding the reins of a horse that was trotting along behind) and so starry-eyed in love (it IS Italy, after all) that they think nothing of making a sudden LEFT turn right in front of you from the vicinity of your right bumper. YIKES! Also, they zip up behind you and hover at the front corner of your car. There must be some unspoken thing they all know, but just make sure you don't make any sudden moves with your car or there might be one you clip. What we finally figured out is that if they are BEHIND the driver/passenger (say behind the front seat section of your car) then don't worry about them, they're watching you. Italians literally never look in their rear view mirror and you don't need to either. But once they've pulled up even with the driver/passenger (so riding along parallel with you, front seat or forward) then make sure someone is watching them because I believe they're counting on you to be watching what THEY'RE doing. Make sense?
But don't let me scare you, Italian driving is mostly a breeze, especially once you're north of Rome.