CORK, June 19, 2014 (AFP) - Ilonen, 34, celebrated his 300th European Tour event by setting a new course record of seven-under-par 64 in superb scoring conditions on the Fota Island course in Cork.
The Finn ended the round with a two-shot lead over the trio of Germany's Marcel Siem and the Swedish pair of Robert Karlsson and Magnus Carlsson.
In contrast, McIlroy, the highest world-ranked player in the field at No 6, finds himself starting the second round sharing 125th place in the 156-strong field.
McIlroy could manage just three birdies and double the number of bogeys in a three-over-par 74 to trail the triple Tour-winning Finn by 10 shots.
Ilonen capped his round by birdying five of his closing seven holes and taking just 26 putts.
"A couple of weeks ago in Sweden I was playing really well and I finished 32nd, and it's still really a mystery how I finished 32nd that week, because I was actually hitting my best shots of the year there," said Ilonen.
"I felt really calm with the putter and today really it's just still there but we still have a long way to go."
And while Ilonen sets his sights on a fourth Tour victory, he also is looking to break into the world's top 50 for the first time in his career.
He is currently ranked 63rd, one place lower than his career high of 62nd in December 2007.
McIlroy, however, paid a price for disruptions to his preparation including losing his clubs and struggling to hit just eight of the 14 fairways and taking 30 putts in a round of 74.
"I've still got a little bit of jet-lag, for sure, coming from the US Open," he said.
"I wasn't able to do much on Monday or Tuesday because the clubs didn't arrive until yesterday so that was a little bit frustrating more than anything else.
"But I did a really good practice session on the range last night after the Pro-Am and was feeling good going out this morning.
"However, I just couldn't really put it together as I was very sloppy with the scoring clubs today, wedges, the short game, the putting. All that was just a little off.
"So hopefully that is my poor round for the week and I can do a better job of that tomorrow and obviously I'll need something in the low 60s just to be here on the weekend."
Leading scores after the first round of the Irish Open at the par-71 Fota Island course on Thursday:
64 - Mikko Illonen (FIN)
66 - Marcel Siem (GER), Robert Karlsson (SWE), Magnus Carlsson (SWE)
67 - Edoardo Molinari (ITA), Matthew Baldwin (GBR), Anders Hansen (DAN), Daan Huizing (NED)
68 - Darren Fichardt (RSA), Gregory Bourdy (FRA), Richard Finch (GBR), Justin Walters (RSA), Ross Fisher (GBR), Adam Gee (GBR), Jake Roos (RSA), Tom Lewis (GBR), Milkha Jeev Singh (IND), Michael Hoey (GBR), Robert Rock (GBR), Graeme McDowell (GBR), Marco Crespi (ITA)