Irwin opened in 3-under-par 67 at Southern Hills in Tulsa, Okla., to take the Day-1 lead as play was suspended due to inclement weather.
However, the 56-year-old went 75-74-76 to finish the event at 12-over-par.
Even more impressive than Irwins Thursday 67 was Tom Kites Sunday 64. The 1992 U.S. Open champion finished the championship tied for fith place at 1-over-par.
Kite is now 9 for 9 in cuts made on the PGA Tour since turning 50.
Kite will join Irwin in this weeks field; as will Jim Colbert, Allen Doyle, Bruce Fleisher, Gary McCord, Arnold Palmer, Gary Player, Dana Quigley, Lee Trevino and defending champion Larry Nelson.
Nelson fired a 6-under-par 66 at Nashawtuc Country Club in Concord, Mass., to better Jim Thorpe by four shots.
Irwin recorded victories at the 1998 and 99 FleetBoston Classic before Nelson took the title a year ago.
This years event carries a purse of $1.4 million with $210,000 going to the winner. It is the final tune-up for next week's U.S. Senior Open.
Full-field and tee times for the FleetBoston Classic