Notre Dame has a contract with the ACC. They are full members in all sports except football, and are required to play x football games vs ACC each year. This doesn't happen by accident.
ND stays independent in football, because amazingly the contract with NBC for 'Notre Dame' football pays more money for their 6 home games than a share of a power conference TV package.
As soon as that is no longer the case, ND will either do the logical thing and fully join ACC, or shock the world and go SEC or Big10.