The 22-year-old Derry City player is due to undergo a medical with the Black Cats on Monday.
McClean had been named in Nigel Worthington's squad to face Faroe Islands this week, having represented Northern Ireland at youth level, but he is now switching to the Republic.
"It was a blow seeing Irish League players being called up before me to the Northern Ireland squad, but having got the call I was relieved," McClean told the Belfast Telegraph.
"However, it's always been my dream to play for the Republic and hopefully that will become a reality now that I'm going to be a Sunderland player."
McClean made 73 appearances for Derry, scoring 18 goals.