The former Manchester United defender has paid tribute to the 71-year-old and believes him to be the greatest of all time
Reacting to the news of the Scot's decision to retire as manager of the Red Devils, Irwin said that it was a "sad day" for the club, but stressed that the 71-year-old's achievements should be celebrated.
"It's a sad day for Manchester United," he said. "It's a day we should celebrate what he's done for Manchester United. It was always going to come as a shock. Without doubt the best manager that's ever lived."
He continued: "He keeps winning, winning and winning. It was always going to come as a shock and he's just won us his 13th Premier League title – his record is just phenomenal. His gathering of trophies has been unbelievable."
Ferguson brought Irwin to Manchester United from Oldham Athletic in 1990, and the defender remained with the club for 12 years, amassing over 350 league appearances before joining Wolves in 2002. The Ireland international won seven Premier League titles with the Red Devils and picked up a Champions League winner's medal during his time at the club.
Irwin described his former manager as "the most honest manager" that he had worked with and praised the Scot's man-management skills.
"He was very honest to work with," he said. "He's the most honest manager I've ever worked under. There were no grey areas with him. You know what you can and can't do on and off the football field.
"He's very understanding as well. For all the psychologists involved nowadays, he's the best of them all. He understands, as you need to, that football is a tough game with different personalities within that game.
"You have to deal with that accordingly and, as a man-manager, I've never come across anyone better. It was a true joy to work under him."