Ilonen carded a final-round 70 at Fota Island to finish 13 under par, one shot ahead of Italy's Edoardo Molinari, with Sweden's Kristoffer Broberg and English duo Danny Willett and Matthew Baldwin another shot back in third.
Graeme McDowell had been hoping to claim a first professional victory in Ireland in front of the massive crowds in Cork, but struggled on the greens and had to settle for a closing 71 and a share of sixth place, his first top-10 finish in 13 appearances in the event.
Ilonen led from the moment he established a new course record of 64 at lunchtime on Thursday and although the record itself only lasted until Willett, aided by a hole-in-one, shot 63 on Saturday, the Finn took a one-shot lead into the final round.
Birdies on the second and fourth took him three clear of the field until Willett carded his second birdie of the day on the ninth, but that was as close as anyone got until Ilonen dropped his only shot of the day on the last, ironically after hitting an iron off the tee for safety.
"It was very much a Martin Kaymer-inspired win," said the 34-year-old. "I sent a message to him last week that I really enjoyed watching the US Open. I very rarely enjoy watching it, I would rather play, but Martin's win inspired me.
"I was thinking about winning this tournament from midday Thursday, basically. Somehow the course looked easy to me and I felt we were going to see some crazy scores.
"I had a low one to start with, and it was something that I was expecting, but I could see from the faces and comments of other players that they weren't expecting it. I had a number in my head all week that I tried to get to and I missed it by a mile."
That number was 21 under par and having fallen eight shots short, Ilonen added: "It's been a hard day's work to keep that lead and luckily no-one really made a run out of the last couple of groups."
Ilonen won the Amateur Cham-pionship at Hoylake in 2000 and was 16th in the Open there in 2006, so was understandably excited by the prospect of returning to Royal Liverpool for the Open next month.
He added: "I've been thinking about it a lot since I qualified off the order of merit last year. I would like to go in between those big crowds on Sunday, so that means I need to be a little bit better than 16th."
Molinari, Willett and Baldwin will also be at Hoylake after securing the three available places via the new Open Qualifying Series, with Baldwin edging out Broberg due to his better world ranking.
McDowell was left to rue a poor putting performance which meant he recorded just one birdie, the former US Open champion also three-putting the fourth for a par 5 and the 15th for his solitary bogey.
"Perhaps one of the worst putting weekends of my career when in contention," McDowell said. "I let a lot slip away yesterday and continued in the same vein today."
English teenager Matt Fitzpatrick, the only amateur to make the cut in the US Open last week at Pinehurst, finished in a tie for 29th in his first tournament as a professional following a closing 68.