Paul Townend had topped the table for the past five years but went into the last afternoon of the Punchestown Festival trailing 123-119, with five booked rides left to turn things around.
His task was made even harder when opening mount Klarc Kent was pulled up and a smooth success for Lossiemouth only delayed matters, before Glengouly was another Townend ride who failed to complete the course.
Townend closed the gap to two when scoring on Kargese but was out of luck on his final mount Olympic Man.
Kennedy, who has struck up a formidable partnership with trainer Gordon Elliott, said: “It’s an unbelievable feeling to have won the championship.
“Paul got close enough in recent weeks to give me loads of sleepless nights but I’m delighted to have hung on. I’ve had a great 12 months, leading the championship for much of the way and riding over 100 winners in a season for the first time in my career.
“There were loads of great days and riding Grade One winners in Ireland on such good horses as Gerri Colombe, Teahupoo, Farren Glory, Found A Fifty, Caldwell Potter and Irish Point was very special.
“A massive thanks to Gordon and all the owners, all the staff in Gordon’s and all the trainers that have supported me this year, my agent Kevin O’Ryan and everyone behind me. Without my family, this wouldn’t be possible.”
Elliott added: “I’m thrilled for Jack. We have been thinking about this day since he was 16!
“Unfortunately, he got a couple of hold-ups each year. Last year, he was clear but got hurt in Naas. We knew this year he was in good nick. He’s a lot stronger.
“I’m so happy. He deserves this more than anyone, with all the injuries that he’s had. He bounces back and that’s his attitude. Hopefully we’ll get another 10 or 15 years out of him.”
Willie Mullins continued his domination of the National Hunt trainers’ championship in Ireland, coming out on top for the 18th time and adding to his memorable title triumph in Britain.
He said: “It’s fair to say that this has been our best season at home and abroad, and it is a credit to our hard-working and dedicated team that we are celebrating again today.
“The horses have been in some order from around November-time. They’ve just been so healthy and even this week, some of them have been putting in their best performances. When you have a healthy horse, they can keep on improving the whole season through.”
Champion owner JP McManus said: “It’s such a great honour to win this title and I feel so lucky. My sincere thanks to all the trainers, jockeys, stable staff and all who contributed so much to my success.
“I have had wonderful days out with my family, grandchildren and friends and the season has given me memories which I will greatly treasure.”
Danny Gilligan was crowned the champion conditional rider, Patrick Mullins was the leading amateur jockey and Jody Townend prevailed in the female version of that category.