Larry Ziegler, Allen Doyle, Larry Nelson, Bob Gilder and Jim Holtgrieve are tied for third place with 4-under-par 67s.
Irwin broke into red figures early Friday with a pair of two-foot birdies at the first and third holes. He played steady with pars through most of the front nine until a seven-foot birdie putt at the ninth gave Irwin a front-nine 3-under 32.
The Senior Tour's all-time wins leader then knocked a 7-iron to eight feet to set up birdie at the 12th and played a beautiful 3-iron to four feet for birdie at 14.
'I played exceptionally well tee-to-green,' said Irwin, whose 66 was his 14th consecutive round under par, the longest streak on the Senior Tour in 2002. 'I left some putts out there. I never got the right pace but I can't complain about this round.'
Irwin might have a measure of revenge on his mind this week. In last year's rain-plagued event, Irwin tied for 41st, his worst finish of the 2001 season.
'I didn't play well here last year,' said Irwin, a three-time winner this season. 'There were some pretty ugly shots I hit but we're seeing a much different golf course this year and the course is still relatively new.'
Marsh, whose last win on the Senior Tour was the 1999 Tradition, also got off to a solid start on Friday, nearly mirroring Irwin's front nine. Marsh holed two 10-foot birdies at one and three and ran home a six-foot birdie at the ninth for his 32.
The 58-year-old collected a birdie at the 10th and nailed a 40-foot birdie putt at the 12th to reach 5-under-par. Marsh took sole possession of the lead with a 12-foot birdie at the par-5 15th, but fell out of first when he landed in a fairway bunker and couldn't get up and down at 18.
Despite the closing bogey, Marsh's 66 marked his lowest round on the Senior Tour in 2002 and gave him a share of the first-round lead for the first time since the '99 Tradition.
'It's nice to get away to a good start,' said the Australian, who won the 1997 U.S. Senior Open. 'I'm probably playing a little better than I've been scoring lately.
'The winner out here usually has a pretty good streak with his putter. A great putter can adjust very quickly. I was trying to make certain I didn't over-read the greens today.'
U.S. Senior Open champion Don Pooley, Jay Overton, John Bland, Dave Stockton, Wayne Levi, Bruce Lietzke, Jose Maria Canizares, Hugh Baiocchi and John Harris share eighth place at 3-under-par.
Jim Thorpe, the Allianz Championship winner in 2001, carded an even-par 71 in the opening round of his title defense and is tied for 40th.
Full-field scores from the Allianz Championship