McIlroy was in danger of letting a promising opening round slip away when he bogeyed the 14th and 15th to fall back to level par, but he birdied the last three holes at Royal County Down to record an opening 68 on Thursday.
That represented a 12-shot improvement on his first round the last time the highly-rated course staged the Open in 2015, and left him two shots off the lead held by England's Todd Clements.
"Any time you can shoot three under around this place you have to be happy," said McIlroy, who is well aware that his record as a professional on home soil leaves a lot to be desired.
"The conditions were pretty tough early on and then the wind seemed to settle a little bit those last few holes and it was nice to take advantage of that and play them the way I did.
"Giving those shots away on 14 and 15, to get more than both of them back on the last three holes was a great way to finish and gives me plenty of momentum going into Friday morning.
"It's nice to get off to a good start in Northern Ireland for a change. I enjoyed being out there, the crowds were great.
"Really pleased with how I started and hopefully I can continue into the next three days."
Denmark's Soren Kjeldsen won a three-man play-off to lift the title in 2015 and was just a shot behind McIlroy on two under after a superb 69.
The 49-year-old is preparing for life on the seniors tour next year, but could sign off in style with a fifth European tour title.
Former Open champion Shane Lowry had to settle for a one-over-par 72, but that represented a good effort after dropping shots on the second and third and then missing the fourth green.
There was little to cheer for the Australian challengers with Jason Scrivener lying joint-101st after an opening 74 while David Micheluzzi ended a miserable outing in the cold and wind with a quadruple-bogey nine at the 18th that left him clutching a horrible card of 79.
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