On a day of strong winds and intermittent showers, McIlroy hit nine bogeys and failed to record a single birdie in a nightmare round at the par 71 Royal County Down course.
The 26-year-old, who sits 150th in the 155-man field, trails leaders Padraig Harrington of Ireland and Germany's Maximilian Kieffer -- who carded four-under-par 67s -- by 13 strokes.
Northern Irishman McIlroy missed the cut at the last two Irish Opens and the BMW PGA Championship at Wentworth last week.
"Off the tee, I felt my game was ok and I put myself in good positions, but then as I got closer to the greens, the worse it got," said McIlroy.
"I didn’t hit very many good iron shots and then from there my putting wasn’t that good... I sort of just lost concentration on the greens.
"So, nothing went right and I was trying hard to birdie the last to try and break 80, but it just didn’t quite happen."
Fellow PGA Championship and U.S. Open winner Martin Kaymer did only marginally better with an eight over par 79.
German Kaymer also failed to score a birdie, had three bogeys and two double bogeys in his round.
McIlroy’s score is his poorest in Europe since a similar round of 80 on the second day of the 2010 Open Championship at St. Andrews.
"I've got to go out tomorrow and try my best and try to claw my way back inside the cut-off line and back into this tournament,” he said.
“It’s disappointing for the large crowds out there, who followed us around this morning, as I wanted to play well not just for myself.
"I am not sure what score I am going to need to get into the weekend but I would love to be here for all four rounds.
"I hope my tournament is salvageable and that will be my goal starting off tomorrow."
Only 13 players broke par while American Rickie Fowler and four-time major winner Ernie Els carded respectable level par scores of 71 to sit within touching distance of the leaders.
(Editing by Tom Hayward)
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