David Moyes' men return to league action against Crystal Palace on Saturday tea-time following a warm-weather break last weekend.
The Reds are currently 11 points adrift of the top four and Irwin insists three points is a must against Tony Pulis' in-form Eagles.
"If we have any designs on finishing fourth, we have to start winning games and putting pressure on teams above us," Irwin told ManUtd.com during his visit to Washington DC to launch the Tour 2014 matches, presented by Aon, against AS Roma and Inter Milan.
"Palace won't be easy though. Tony Pulis has done unbelievably well since he took over [in November] and has got them playing with a lot of confidence.
"I'm sure many people wrote them off two or three months ago, given how they were struggling since stepping up from the Championship. But since he's taken over he's done a fantastic job and it'll be a really tough game for us. But it's one we have to win. It's as simple as that."