McCarthy told reporters after the game that he was "happy enough" with the performance, despite the team's run without a win extending to six games.
"I've just been listening to [Brentford manager] Dean [Smith] saying they deserved one like that, a pretty soft one, so that tells you everything when the home manager is saying that – it wasn't a penalty," the coach said.
"I can't do anything about it. We played well today I think. I was happy enough with the performance. I knew it was going to take something like that to win the game.
"hey were getting frustrated. Our only downside is that we weren't good enough with the ball. We could have been better and made the most of our opportunities to hurt them. It hinges on a poor decision in my view."
Ipswich face Barnsley at Portman Road in their next outing on Tuesday.