Giggs scored in the 2-0 win in what was his 999th appearance.
Irwin told MUTV: "Sir Alex trusts him, he knows it's going to be a tough game and yet he sticks him in the centre of the park and he scores a great goal. He's a fantastic player, he really is.
"To play 999 games is unbelievable. He had problems with his hamstrings in his 20s but then started doing yoga. That sorted it out. Obviously the manager's been careful with him over the last eight or nine years, but he's an incredible athlete. There's not an ounce of fat on him.
"He looks after himself, he trains as hard and intense as he plays on a Saturday. Young players can learn bundles off Giggsy and Scholesy through the way they train and the way they are in the dressing room. Ryan is a very big character in that dressing room - he's not quiet. He's a great professional and played fantastically well on Saturday."