McIlroy (25) joins Brassey as the only bowler to have won the prestige invitation event three times since it was first held in 1990.
"It's awesome to be up there with Rowan and it's pretty cool to have a third portrait of myself on the clubroom walls," McIlroy said.
"I'm stoked. I know how tough the tournament is to win."
It was a game that tested the temperament of both bowlers, especially when the weather added its tricks to the mix.
Heavy rain flooded the green on the 20th end and play was halted for 42 minutes with Grantham (39) leading 16-13. When it stopped, the green-keeping staff swept the water off the green and play resumed.
McIlroy took control after the break and added eight shots on the next four ends to lead 21-16.
"I had to make sure I won the first end to get back the momentum," McIlroy said.
"I was three points behind and would have been in big trouble if Tony had won those ends."
McIlroy had a five-shot advantage after 23 ends and was close to the magical 25 when he led 23-19 after 27 ends. He was within one point of the $5000 winner's purse when he led 24-20 after 29 ends.
"Shannon picked the weight better than I did after the rain," Grantham said.
"I just tried to hang in there."
Grantham was not ready to concede and fought back to win the next four ends to level the scores at 24-24 after 33 ends of the marathon match.
"You have to keep fighting to the end. Shannon's draw play was outstanding and I had to get as close as I could."
It was McIlroy's temperament that was tested on the last two ends.
On the 33rd end, Grantham drew accurately and had four bowls on the head. McIlroy drew second shot with his last bowl and was still in the game.
Grantham put the pressure on McIlroy on the final end when he pushed out his bowl with his fourth bowl and had the shot.
McIlroy's temperament was tested.
"I just went through my normal routine," he said.
"At the end of the day it was just another bowl and I just had to make sure I wasn't short and draw the shot."
Grantham's ability with the fourth bowl gave him the early advantage and kept him in the game before the long break for rain.
In the first 20 ends his fourth bowl turned a possible deficit of nine shots into four shots for himself on seven ends.
The draw play of both bowlers was superb - Grantham had 12 touchers and McIlroy seven - but their upshots and drives were off target. McIlroy hit only five of his 11 shots and Grantham two from five.
In the plate final, Andrew Kelly (Christchurch) took control from the start and led 14-4 after 10 ends and went on to beat Raika Gregory (Waikato) 21-10
North East Valley
Invitation singles
Semifinals: Shannon McIlroy (Nelson) 25, Shaun Scott (North East Valley) 19; Tony Grantham (Auckland) 25, Ali Forsyth (Sydney) 23.
Final: McIlroy 25, Grantham 24.
Plate semifinals: Raika Gregory (Waikato) 21, Andre Smith Blenheim) 14; Andrew Kelly (Christchurch) 21, Pat Houlahan (Alexandra) 9.
Plate final: Kelly 21, Gregory 10.
