Irwin, a two-time runner up here, finished at 8-under-par 134 for his Champions Tour record 42nd title. The victory was his second of the season and gave Irwin multiple wins for the 11th consecutive year, by far a tour record. He also becomes the fifth-oldest player to win on the senior circuit.
'It's been a very funny week. It's taken me three days to play 18 holes,' said Irwin, who has 102 top-three finishes on the Champions Tour. 'The real momentum for me, I played the back nine in my first round so spectacularly and then had to sit on it for a long time and really never got that momentum back, but neither did anyone else.'
The event was reduced to 36 holes after bad weather on Friday and Sunday forced lengthy suspensions of play. Twenty-two players in the field completed their final rounds on Monday.
Hatalsky, who completed his second round on Saturday, finished at minus-seven. McNulty tried to defend his crown as he birdied two of his final four holes to tie for second place after a round of 68. Vicente Fernandez and Tom Wargo shared fourth at 6-under-par 136.
Irwin, who played the back nine first in the final round, began his day on the 16th hole at the TPC of Tampa Bay and one stroke behind Hatalsky. Irwin parred his first two holes to remain right there.
The 59-year-old nearly holed his second shot at the par-4 18th, as his second shot stopped within 2 feet of the cup. He kicked in that birdie putt to grab a share of the lead.
Around the turn on the front nine, Irwin moved atop the leaderboard with a birdie at the par-4 third. He parred his final six holes, including a tough two-putt par at the last, to hang on for the win.
'I just never got the pace of the greens this week,' Irwin said. 'I had some putts on what was my back nine today that I could have kind of sealed it up, but I just couldn't get the ball to the hole. The greens are a little slower this year. With the rain, I just didn't get the right tempo going and that's what it is. I didn't transition very well, although it was good enough.'
McNulty, who posted a final-round 68 for a one-shot win over Larry Nelson in 2004, got his round going at the par-4 16th, his seventh. He birdied that hole to move to 5 under.
The Zimbabwe native parred his next seven holes before making a late surge. McNulty birdied the par-3 sixth to get within two of Irwin. After two pars, he needed to hole his second at the last to force a playoff.
McNulty's second shot to his 18th, the ninth, bounced over the hole and came to rest five feet past the cup. He drained that putt to share second.
Wayne Levi, Jerry Pate and Don Pooley shared sixth place at 5-under-par 137. Tom Kite, D.A. Weibring and Mark McCumber finished one stroke further back at minus-4.