To be completely pedantic, the password is not encrypted at all. It is tansformed using a one-way cryptographic hash function that has been designed to have a low (close to zero) incidence of collision for any data. Examples include MD5 and SHA-1 algorithms.(A useful background at http://www.schneier.com/essay-074.html)

If the Data was encrypted then it would have been transformed into another form in such a way that it could be decrypted at a later time. As such, you can't have irreversible encryption.