McCarthy made his first start for the Republic of Ireland in their World Cup Qualifier against Kazakhstan, a sign of the 21-year-old's progression according to Martinez:
Martinez told the Wigan website: "James is ready for these commitments now and to form partnerships on the pitch at international level; he showed a good understanding with Glenn Whelan in the centre midfield, both being very flexible which they had to be in turning round the game against Kazakhstan.
"He showed great composure in his performance and was outstanding on the ball, bringing stability to the team with his partnership with Whelan in the midfield.
"His ability doesn't surprise us anymore but for his country to see what he is capable of at such a young age will be very pleasing I'm sure.
"He had a difficult start to last season, not able to find continuity because of injuries and setbacks in the previous campaign.
"The work he's done this summer was fantastic, helped him to relax and helped him to prepare for the new season, starting with so much maturity.
"He is a unique player, with unique character and I'm sure he'll continue developing into a top, top player at world-class level."