The former left-back, who was speaking to the club's official television channel, has also claimed that "it was fantastic" to work under Ferguson, who announced his retirement on Wednesday.
"It was a bit of a bubble, you had to group together as a team and the team spirit there has been absolutely fantastic. Even more so today when you have the squad size you have and to keep them all happy as he does is fantastic. It was fantastic to work under him," he told MUTV.
Irwin made more than 350 appearances for the club.