McGinnis birdied the final hole of regulation of the senior tournament to tie Irwin, a five-time winner this year and two-time defending champion of this tournament.
The birdie on 18 gave McGinnis a 5-under-par 67 on the 6,773-yard Nashawtuc Country Club course and an 11-under 205 total. Irwin, who has a combined 45 victories on the two tours, had a final-round 68.
Both McGinnis and Irwin parred the first playoff hole, No. 18. McGinnis then won the $180,000 first prize when he hit his 6-iron tee shot on the 170-yard 17th within 4 feet and made the putt after Irwin missed a 12-footer for birdie.
McGinnis, 51, played the extra holes with the confidence of a successful veteran, not someone who won just $32,000 during over 20 years on the PGA Tour.
"Once Hale missed that putt, I started to grin he had just lost," McGinnis said of the second playoff hole. "There was no way I was going to miss a 4-foot straight uphill putt."
"If he had made the putt, it might have been a different story, but to be honest I never expected to be in position to win until I was in the playoff."
McGinnis, who was tied for the lead after the first round, started the final round one shot off the lead shared by Irwin, Sigel and George Archer.
McGinnis said he didn't expect to win after Irwin got hot and birdied Nos. 9, 10 and 11 to take a one-shot lead.
"When I made a birdie to go 10 under on 10, I expected to be leading, but when I saw the scoreboard and Hale was leading, I felt it would be just another good pay day," McGinnis said.
"I just couldn't get anything until 18 when my chip shot landed a foot from the cup."
Irwin said his luck ran out after the string of birdies.
"I thought I had things going pretty well, but then I didn't make another birdie for nine holes, including the playoff," he said. "On a course like Nashawtuc you have to make birdies and I didn't."
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