McCarthy declared for the Republic of Ireland through his late Donegal-born grandfather in January 2007. He has played for Ireland at U17, U18, U19 and U21 levels.
Wigan manager Roberto Martinez said about McCarthy on www.wigantv.co.uk
"There were several club chasing James and it is fantastic he has chosen Wigan. He had a very difficult decision to make, and fair play to him, he took his time to weight up his options, take advice and make up his mind.
"That shows a huge amount of maturity for a boy of his age, but then maturity is probably the one word I would use above all others to describe him. "He has the ability to become a big player in the future and it is great to have him on board."