Notre Dame and the ACC have a very complicated relationship right now. For every sport other than football and ice hockey, Notre Dame plays in the ACC. In football, the team is independent, which they have been since the start of the program. Notre Dame and the ACC have a contract that allows Notre Dame to schedule five ACC games a season.
The Fighting Irish did that, and this past football season, their first opponent was Miami, a team that ended up getting into the College Football Playoff over Notre Dame. That has certainly been the biggest talking point of the bracket reveal was the Irish getting snubbed in favor of Miami.