Is it permissible to fast 30 days to make up for Ramadan, in the event of not remembering the number of days that must be made up due to menstrual irregularities?

You are not required to make up thirty days, because menstruation does not last that long. If the number of days of bleeding you experienced in Ramadan exceeded the maximum duration of menstruation (fifteen days), then you are considered to be experiencing Istihada (non-menstrual vaginal bleeding). In this case, it is sufficient for you to make up a number of days equal to your usual menstrual period before the Istihada, as the Prophet, peace and blessings be upon him, did with Umm Habiba bint Jahsh. If you have missed prayers or fasts and do not know their exact number, then make up what you believe most likely clears your responsibility, or what you are certain clears your responsibility.