Hike the Gap of Dunloe, don't drive it.....its a great hike (more of a walk). Avoid the horse carts. They're cool to look at but aren't very comfortable (at least they didn't appear to be so).
Take a rain jacket (not an umbrella). Walking the gap we encountered practically all 4 seasons.
Pro tip for the Gap: Have your hotel arrange a package that drops you at the jump off point and retrieves you on the small dock at Loch Leane. Take a guide boat across the Loch to Ross Castle. The package will also include pick up at Ross Castle and delivery to your hotel. Just ask about a package like this, the hotel will know what you're trying to do and arrange it through the locals.
As for driving, in general, Otis is correct. I've never encountered more courteous drivers in my life. Wife and I would joke that when we encountered cars coming the other way on a one lane road it's almost like they new we were Americans and they made an effort to get out of our way. As said above, watch the hedges....behind the shrubbery is a stone wall.
Don't be intimidated by all the warnings in this thread. Heed them, but don't freak. Just do it....