"I'm thrilled Colin has joined us," McCarthy told the official Sunderland website.
"There was some strong competition for his signature, including from some Premier League clubs, so it's pleasing to have secured the services of a player of his quality.
"I know Colin well and I'm fully aware of his strengths. He'll be a great addition to our squad.
Healy said: "I'm delighted to get the whole thing sorted and be joining Sunderland.
"I think this will be a great club to be part of, I know from my time with the Republic of Ireland that Mick is a good manager and someone I can work with."