The 25-year-old has been without a team since being released by the Hammers at the end of last season.
While refusing to confirm whether Collison will be offered a deal at Portman Road, McCarthy admits that he has been impressed by the Welshman.
"He's certainly got real quality. He wanted to get himself fit," McCarthy told BBC Sport. "He lives half an hour away, I told him to come and train with us. Who knows? Something might come of it, or something might not."
Collison had been training with Queens Park Rangers before joining up with Ipswich.