The world number four trails eight strokes behind American Daniel Im and Frenchman Benjamin Herbert.
The world number four trails eight strokes behind American Daniel Im and Frenchman Benjamin Herbert, who shared a new course record with their eight-under-par 64s.
As both the host and defending champion, McIlroy was also upstaged by his two playing partners -- Jon Rahm, who eagled the seventh hole and also hit six birdies to share third place on seven under par 65, and world number two Hideki Matsuyama, who signed for a 67.
"Nothing was going my way and it was hard to get anything going in the right direction. It was like I was swimming against the stream all day," said McIlroy.
The 28-year-old Northern Irishman, who pulled out of the PGA Championship in May because of a rib injury, continued to struggle both off the tee in hitting just six of 14 fairways and then again on the greens in taking 33 putts.
"The course was out there for a good score especially when you see what Jon and Hideki managed in shooting five-under and seven-under and, as I said, it just felt like I was going backwards," continued McIlroy, who made an early exit from his home event for three years running before snatching victory at the K Club in 2016.
"It was very frustrating and just one of those days where I hit the ball very well and just couldn't get any momentum or anything going."
"Hopefully, I can get out there tomorrow morning and make some birdies."
The 32-year-old Im had raced to six under through just eight holes but then managed just two further birdies in his next ten holes before being joined by Herbert, 30, who capped his round with three birdies in succession from his sixth and then four in a row from the 13th.
The effort equalled Herbert's lowest score on the European Tour with six prior scores of 64 including two last season.
"I practiced a bit actually last week in France, and I found some keys I think with my coach, and it worked today," he said.
"I holed some ones very important, also, the four birdies in a row, a few long putts. It's okay. I played very good. Only missed one green, no bogey, it's still good."
"We've been a bit lucky I think also with the conditions as the wind stopped near the end, so it was just perfect today."
Unlike McIlroy, Rahm is no stranger to the course having competed in the 2014 British Amateur when Portstewart hosted a qualifying round ahead of the final matches at nearby Royal Portrush.
"It was extremely fun to start the way it did perfect tee shot and the second shot, and that birdie putt had a really calming effect," said Rahm.
"Overall, it was just a great day. The atmosphere was so good. My attitude, it was probably the best it's been all year. I was positive all day."
"I kept my routine going and just stayed calm and the result showed how good it was."
And Justin Rose's return to the event after a seven-year absence began on a bright note with an eagle and three birdies also in his bogey-free 67 to be among 19 players sharing ninth place on five under.
"Today was the day you relish with that perfect ten, 15-mile-per-hour breeze where the course plays tough but still if you play well, there is a score there," said Rose.
"But then I think today was probably a bit too benign to really test your game. Nevertheless, it was fun."