Im picked up eight birdies from his first 14 holes and was unlucky to just miss out on birdies at 15th and 16th holes to extend his lead further.
The 32-year-old, who sat 109th in the Race to Dubai before the tournament, brilliantly holed a tricky par putt at the 18th to ensure he held the lead outright until the final groups finished their rounds.
Then, as the first round drew to a close, Hebert finished with four birdies in his final six holes to join Im on eight under at the top of the leaderboard.
Defending champion and home favourite McIlroy began tentatively, dragging a putt past the hole at the third to card a bogey, before finding bunkers at the 12th and 13th, only to save par on both occasions.
The rest of the day remained a battle for the world number four as he finished with a disappointing 72, missing putt after putt as he parred the final 11 holes -- his longest run of level scores on the European Tour this year.
"I don't think we are going to get this course much easier," McIlroy told Sky Sports. "There was so many scoring opportunities out there.
"I gave myself a few chances, but I just couldn't find any momentum. I tried my hardest, but it was just one of those days where I couldn't get going."
All eyes were on McIlroy's threeball at Portstewart, but it was world number two Hideki Matsuyama and world number 11 Jon Rahm who fared much better than the Northern Irishman.
While Matsuyama carded a more than respectable five-under-par 67, three off the lead, it was Rahm who shone with a seven-under-par 65 despite missing birdie chances on the final three holes.
The English pairing of Matthew Southgate and Oliver Fisher held the clubhouse lead early on, but sit alongside Rahm on seven under.
Lee Westwood endured a miserable afternoon, shooting a three-over-par round of 75.
(Writing by Pete Hall, editing by Neil Robinson)
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