McIlroy was two down with four to play in his semi-final

The match was level after 13 holes but the 16-year-old birdied 14 and 15 to take control of the match and the contest ended on the 17th green.

Earlier, the Holywood youngster had fought back to beat England's David Horsey at the 21st in the semi-final.

O'Hanlon beat Scrabo's Andrew McCormick by one hole to reach the final.

McIlroy's victory means he becomes the first player since Niall Goulding 15 years ago to retain the title.

The 16-year-old shot six birdies in the decider as opposed to O'Hanlon's three.

McIlroy had shown remarkable composure to set up a final meeting with the Curragh clubman by coming from two down with four to play to force extra holes against the Englishmen.

The defending champion then holed a 10-foot par putt to keep the game alive at the 20th.

I just had a knack on the greens all week Rory McIlroy

Earlier after winning the 15th with a par to cut Horsey's lead to the minimum, the Holywood teenager showed real mettle to sink a 15-footer for par to level the match on the 17th green.

McIlroy looked in trouble when he bunkered his approach at the 20th but he splashed out to 10 feet and calmly holed the putt.

Horsey missed the fairway on the third extra hole and was unable to match McIlroy's two-putt birdie four.

In the other semi-final, McCormick's hopes of claiming his first amateur major were ended when O'Hanlon hit his approach to six inches at the final hole.

Following his afternoon triumph, McIlroy acknowledged that brilliant putting had been the key to his success.

"I just had a knack on the greens all week and that is what won it for me," McIlroy told BBC Sport.

"No disrespect to Paul who is a really good player but the semi-final put me in really good stead for the final as Horsey and I had been joint-favourites."

McIlroy heads to New Zealand on Wednesday where he will represent a 12-man European team in the Sir Michael Bonallack Trophy from 26-28 April against an Asia-Pacific line-up.