Not the easiest of roads to travel on, quite windy and dark. At that hour of the day, and depending on the weather, it could be very slow. I'd allow at least 45mins to get to the airport, but arriving at 630 should be fine, Cork airport check in is usually quiet at that time of the morning (unless you're going Ryanair and have bags in which case it might be easier to start queueing the night before )