The four-time major champion's challenge faltered from the off as he struck his tee shot at the first out of bounds in posting a quadruple-bogey eight.
McIlroy regained his composure to get back to three over par before an equally damaging finish as a double bogey at the 16 and triple bogey at the last left him with a huge task on Friday just to make the cut.
"I guess when you play your first and last holes in a combined seven-over par, you're sort of starting on the back foot," said McIlroy.
Huge crowds gathered round the first hole for a glimpse of the local hero as the Open returned to Northern Ireland for the first time since 1951.
However, McIlroy played down the impact of pressure on his terrible start.
"I was nervous on the first tee, but not nervous because of that (playing at home). Nervous because it's an Open Championship. I usually get nervous on the first tee anyway, regardless of where it is."
McIlroy slipped to five over with another bogey at the third, but repaired some of the damage with birdies at seven and nine.
Six pars followed before the wheels came off again with a three-putt from six feet on the 16th.
"The one that I'm disappointed about most is the little short putt on 16. That was inexcusable," added McIlroy.
"Tee shots like the first happen, you can get one riding on the wind a little too much, that's fine. But lapses of concentration like that, I feel like I've done a really good job over the last few years of being more with it and realising just keep a cool head.
"There I didn't. I sort of hit it on the run and missed it. And if I look back at today, it's probably the shot I'm disappointed about the most."
McIlroy hit the course record of 61 at Portrush prior to its overhaul for the Open when he was just 16 and knows he will need some of that magic tomorrow to ensure he hangs around for the weekend.
"I definitely think if I can put the ball in the fairway tomorrow I can shoot a good enough score to be around for the weekend.
"Obviously I'm pretty sure anyone starting with a 79 in this golf tournament doesn't think about winning at this point.
"But I think I can go out there and shoot something in the mid-60s, be around for the weekend, and then try to play good from there."