McIlroy carded two birdies and a bogey in a steady opening nine before picking up a shot on the 12th.
But after missing a birdie chance on 13, he cut a frustrated figure as back-to-back bogeys dropped the four-time major winner back to level par.
However, instead of letting the round slip away, McIlroy finished with a treble of birdies to give himself a boost of momentum ahead of an early tee time on Friday.
“Sixteen and 18 are really good birdie chances,” said the 2016 champion.
“After the two bogeys on 14 and 15 I thought if I can get those two back it would be a good start so to get one more back with the birdie on 17, it’s a really nice way to start.”
McIlroy, whose Irish Open win came at the K Club, added: “I felt like I controlled my ball flight well.
“I’ve been working on my swing these last couple of weeks and it felt a little better. I’ve probably struggled a lot in left-to-right winds this year so to control my ball flight a bit and test it out there today, it was good to see I could do it when I needed to.”