The duo have formed an important partnership for Roberto Martinez's side since they both joined the Toffees last summer, and Barry admits that the 23-year-old is even better than he expected.
"I've played against him and known he was a good player, but until you play with someone and train every day you don't really know," the England international told reporters.
"James, considering he is so young, he is so mature and knows exactly what to do in every situation.
"James for me is up there and in terms of playing alongside, and influencing games he is up with the best I've played with already."
Barry's partnership with McCarthy could end if a move to Everton is not agreed when his deal with parent club Manchester City expires in the summer.