We always know that ND is a special case. By all rights the Irish should have joined the Big10, who really wanted them as a full football member, and would have established a number of tremendous geographic rivals. But the ACC made ND a better deal. The Irish had there own tv contract with NBC which paid them many millions of dollars.
Geographically, ND joining the ACC made about as much sense as UConn joining the AAC. But in exchange for playing I believe 6 football games per year against ACC foes as an independent, ND gets to remain an independent with their own tv contract while gaining the benefit of a P5 conference affiliation for every other sport.