Notre Dame has agreed to join the ACC as a full member in all sports except football. It will play five games a year against ACC teams, though. Apparently Clemson President James Barker was a driving force behind the decision to bring Notre Dame into the league.
So once every three years, an ACC team will get the honor of facing Notre Dame in football, but the game won't actually count in the standings. (And once every six years, they'll play in your favorite ACC team's stadium.) The Irish certainly know how to leverage their brand.
Regardless, the mega-conference trend is proceeding as I have foreseen.