The veteran full back, now battling for Premiership survival with Wolves, has watched with interest as his former employers have struggled defensively in recent weeks.
The United rearguard have shipped ten goals in their last four league outings after losing Rio Ferdinand to his eight-month ban.
In that time The Red Devils have lost valuable ground to Arsenal in the title race, and Irwin feels that they must tighten up at the back or give up hope of reeling in The Gunners.
"Life is tough at the top and United's defenders have got to step up soon," declared the 38-year-old.
"A whole season's work could go down the pan in the next three weeks if they don't tighten up.
"You can't go on hoping to score one more goal than you concede in every match - eventually you'll get caught out and there are big games on the horizon that United simply can't afford to lose.
"The defence is missing Rio Ferdinand and too much has been asked of Wes Brown too soon.
"United will work their way through what has happened in the last few games, but it has certainly cost them valuable ground in the title race.
"Five points is a really big gap to close on an Arsenal team that is playing really well.
"Arsenal haven't lost in the league this season and are bound to have a sticky patch at some stage.
"But the question is whether United will be close enough to take advantage when Arsenal do hit trouble."
