Carlos Edwards' late winner secured a priceless three points that lifted McCarthy's team four points clear of the relegation zone and put a huge dent in Bolton's play-off aspirations.
Wanderers arrived at Portman Road on the back of five straight league wins, and McCarthy said: "It was a huge result and a great three points. Teams don't come much more in-form than Bolton.
"To be fair, we've done alright against the form teams - we've struggled against the likes of Barnsley, Peterborough and Bristol City.
"I don't know why that should be, but we've certainly progressed. We've got better and become harder to beat.
"We had a really good first half. They had a spell in the second when they got on top, but when we changed our shape and stemmed the tide we deserved to win overall."