Nothing in Saint Malachy’s prophecies suggest that the last pope he lists will be the last pope. “Peter the Roman” was added by a later hand to St. Malachy’s, and even if it were legitimate, there’s nothing to suggest that “Peter the Roman” immediately follows “The Glory of the Olive.”
The "prophecy" was discovered more than four centuries after St. Malachy's death.
Many consider it to be a forgery for two reasons:
1. The popes prior to the discovery fit very well into the predictions. The popes after the discovery require a lot of latitude to fit within the predictions.
2. Some of the prophecies only apply to anti-popes.