The thing is to relax and not worry about traffic or getting around. ERI is a bit like riding in Europe as the EU has really made a mark on the road markings and signs.
The roads as said above are more like small dual carriageways down to small rural roads. As the EU cash has come in a lot of the roads have improved and easy enough to get around.
People are fantastic and very welcoming so if you ask for directions/help not be an issue. Make sure you get a cheap sat nav or something as worth its weight in gold when in a strange place.
My only gripe with being over there is the cost once you head South from NI but maybe me being a tight Scot lol
Go and have a fantastic time in a great country