Irreversible Moves can not be reversed. Thus, they are either captures or pawn moves . Making those irreversible moves, resets the halfmove clock inside a position object to zero, concerning the fifty-move rule. Quiet and otherwise reversible moves, which lose the castling rights, that is rook- and king-moves from their initial square, including castling itself, are irreversible in the sense to reverse the same rights - since once a castling right is lost, it is lost forever, and reset the position index to determine repetition of positions. However, those "irreversible" moves don't reset, but increment the halfmove clock  .
- Irreversible (disambiguation) from Wikipedia
- Irreversible process from Wikipedia
- Point of no return from Wikipedia
- Computerschach - Eine Wette, die ich gerne verloren habe by Horst Wandersleben (German)