McIlroy, who won the 2011 US Open and 2012 US PGA by eight shots and was six clear going into the final round of the Open at Royal Liverpool a fortnight ago, had to dig deeper than he has ever had to in securing his fourth major.
His performance on the back nine, which he began two shots off the lead, was especially pleasing for Smyth who was delighted to see his fellow Irishman show that he can find his best from in clutch situations.
"I was one of those guys who stayed up and watched every shot. He showed a new side to him," Smyth said on Breakfast on Newstalk.
"In the past he has won a lot of these tournaments from the front as he did with the open a few weeks ago and people were saying that he might not have the steel in him down the stretch when things got a bit tight but they have been proven wrong again.
"He went three behind at one stage and not only did he win, the way he played the last nine holes, he played unbelievable golf.
"He produced the shots when he needed, there was only one player to hit the 10th yesterday (in two) and that was Rory McIlroy and he happened to make an eagle and from then on he hit every iron shot straight at the flag.
"It put just enough pressure on the guys in front to falter a little and that's what happened.
"We have been saying for years that this guy has this sort of talent but the question was can he do it under the heat? Can he do it under pressure? Can he deliver in a dogfight? Rory doesn't mind getting down and dirty, he proved it yesterday."