Irwin (as Mike Rotunda) joined the World Wrestling Federation as part of the U.S. Express (with his cousin, Barry Windham) in 1984. They won the WWF World Tag Team Championship twice, firstly from Dick Murdoch and Adrian Adonis in January 1985. The US Express' most notable feud was with The Iron Sheik and Nikolai Volkoff, to whom they lost the title at the first WrestleMania. The US Express regained the belts in June 1985, but lost them two months later to Brutus Beefcake and Greg Valentine. Windham left the WWF soon after. Rotunda continued to wrestle in singles matches until he left the WWF himself, in early 1986.
He briefly returned to the WWF in the fall of 1986 to team with "Golden Boy" Dan Spivey as a new version of "The U.S. Express", but they were more or less used as a jobber team against up and coming teams such as Demolition in house shows. They were not considered top contenders.
In the WWF, Rotunda became Irwin R. Schyster (or I.R.S.). Schyster had a heel "tax-man" gimmick (he was portrayed as a former IRS tax collector from Washington, D.C.), and harassed the faces and fans, scolding them to pay their taxes. He was, however, a sound technician in the ring, and a serious challenger for Bret Hart's Intercontinental Championship. He had a short-lived feud with The Big Bossman in the fall of 1991, accusing the Bossman of being a "tax cheat".
In February 1992, he formed the team of "Money Inc." with Ted DiBiase, and won the WWF World Tag Team Championship a further three times. Money Inc.'s first title reign was at the expense of the Road Warriors.
After Money Inc. was disbanded (due to Ted DiBiase retiring from wrestling to become a manager), Schyster went back to singles competition. He feuded with Razor Ramon, and later with Tatanka, whom he accused of failing to pay a gift tax on a "sacred headdress" he received from Chief Jay Strongbow. He then joined DiBiase's Million Dollar Corporation stable, often teaming with fellow member Bam Bam Bigelow, in an unsuccessful effort to regain tag team gold. Schyster later refocused on singles wrestling, feuding with The Undertaker in a long rivalry, with both gaining momentum leading up to their clash at the 1995 Royal Rumble. This included I.R.S. defeating many jobbers on WWF Monday Night Raw, repossessing the headstone of a child, and interfering in a Casket Match between The Undertaker and Yokozuna. When the two eventually squared off, the Undertaker was victorious following a chokeslam, but Schyster stole his urn after the match. I.R.S. then competed less frequently on WWF TV, his final two appearances being a loss to Savio Vega in a King of the Ring qualifying match in June 1995, and as a lumberjack the following month at In Your House 2: The Lumberjacks. Schyster then left the WWF.
Rotunda was rehired by WWE as a road agent in 2006, and has made guest appearances as Irwin R. Schyster. One such appearance was on the August 6, 2007 edition of WWE Raw; Mr. McMahon was discussing his IRS troubles with Jonathan Coachman, and when the conversation ended, Rotunda lowered the paper covering his face and revealed himself.
Rotunda appeared as I.R.S. on the December 10, 2007 15th Anniversary edition of Raw, winning a 15 man Battle Royal, only to be paid by his former tag team partner Ted DiBiase to eliminate himself and give DiBiase the win.
On the March 10, 2008 episode of Raw, The U.S. Express were to have a rematch from WrestleMania I against Nikolai Volkoff and The Iron Sheik. However, the match was interrupted before it could begin by diva Jillian Hall, who offered to sing "Born in the U.S.A." for them; she was given an airplane spin by Rotunda.
Rotunda appeared in the segment "Top Rope Theatre" on WWE.com, on February 19, 2009, talking to Kelly Kelly. In this exclusively online storyline, he played a heel character once again and his arch enemy was "Hacksaw" Jim Duggan.
I.R.S. appeared on the September 7, 2009 edition of "Raw," of which Bob Barker was the guest host. Rotunda was a contestant in a pricing game (similar to the "One Bid" qualifying segment of The Price Is Right) along with Santino Marella, Jillian Hall, and Chris Jericho, bidding on a "Best of Smackdown" DVD. Rotunda overbid $50.00 (including tax); the DVD's actual retail price was $18.99. Chris Jericho won the pricing game with a bid of $1.00 (as erroneously proclaimed by Bob Barker). I.R.S was, later that night, one of the contestants during the second round of the pricing game. A. J. Pierzynski of the Chicago White Sox filled in the vacated spot Chris Jericho held. This time the bid was for a travel package to WrestleMania XXVI in Glendale, Arizona. Rotunda bid $2,000.00 for the package. The actual retail price was $1,247.00, making Santino the winner after a bid of $1,200.00.
He appeared on an episode of Monday Night Raw as a lumberjack in the "'80s Legend Lumberjack Match", in which Christian defeated Ted DiBiase, Jr.
On the May 5, 2010 Smackdown, he helped throw Drew McIntyre out of the arena.
On June 7, 2010, I.R.S appeared on Raw in a comedy segment. He announced he had taken Jerry Lawler's crown, because Lawler had not paid his taxes. On the June 28 Raw, he was one of the four people who entered the ring to celebrate Ricky Steamboat's career, but were attacked by the Nexus.
Rotunda appeared on the April 9, 2012 edition of Monday Night Raw. He, along with numerous other WWE officials and superstars, attempted to break up a brawl between Brock Lesnar and John Cena.