Ashton bounced back from a first-half sin-binning to strike twice in the second and help Sarries open their account with a bonus-point win.
"He brings something different, he's never far away from where the action is, particularly when we've got the ball," said McCall.
"He pops up in unusual positions, which is exactly what we wanted. We are hoping he will take our attacking game forward.
"He played really well today and a lot of the others did too."
Ashton made his feelings clearly known after referee Wayne Barnes brandished a yellow card for his late challenge on fly-half Steve Shingler.
But McCall admitted: "It wasn't his finest hour. He didn't use his arms in the tackle. It was an indisciplined thing to have done and I've no complaints about the decision."
The Saracens boss revealed that Ashton had shown his enthusiasm by reporting for pre-season training a week early to integrate himself with his new club.
He said: "He's got his own reputation but we've found him very good so far. I think some of the reputation he has is unfair.
"He just wants a new start and he's chucked himself into the club.
"He came a week early to get to know the people he didn't know already and the players appreciated that."
McCall, delighted with his side's opening performance, added: "In the first half we played a little bit within ourselves. We had some opportunities which we kicked away.
"We needed to up the tempo in the second half and I thought we did that very well."
Brian Smith, the London Irish director of rugby, lamented: "It was a tough afternoon, particularly disappointing for us.
"We came into the season off a very solid win down in Cork so we came into this in a good space. Our preparation was very good.
"We probably needed to pick up all the points on offer in the first half and stay in touch. We didn't do that so when we conceded the first try it was a big margin.
"That hurt us a little bit and put us under a lot of pressure.
"We're disappointed but we accept and understand that's the price you pay when you're trying to build things and get better.
"We have a week to put things right before our first league game. We're a very young group and we're just getting used to each other."
He added: "It's been a kick in the pants today. I think we're a better side than the scoreboard showed. As a collective group we'll front up (against Gloucester) at the Madejski Stadium next week."
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