Hirscher chalked up a combined two-run time of 1 minute, 56.17 seconds on the bumpy course, nipping second-place Andre Myhrer of Sweden, the reigning slalom champion, by 0.57 seconds.
Hirscher credited pre-race scouting of the course with getting him through the rough patches on the Slieme mountainside. "I like to know exactly where the ruts are," he said.
Matt Mario of Austria placed third and David Chuduonski was the top U.S. finisher with a seventh place.
Jens Bygmarrk of Sweden led after one run but got caught up in the rough conditions and failed to finish his second run.
"The difficulty was the fog," observed U.S. coach Sasha Rearick. "In the first run, a groove developed a little bit late in the turn. It wasn't really that big, but the guys couldn't see it at all and they were getting caught by it because it was in a different spot on the turn than typical."