What would happen if an irresistible force (i.e. a cannonball that cannot be stopped by anything) met an immovable object (i.e. a wall that cannot be moved by anything)?

Nothing would happen because the situation is impossible. The term irresistible force and immovable object cannot coexist. If a force is irresistible, there cannot be an object that is immovable, Conversely, if an object is immovable, there cannot be a force that is irresistible. It's as absurd a situation as two people each being taller than the other.