The two verbs are virtual opposites. To prescribe something is to recommend or authorize it: The doctor prescribed a course of antibiotics; The laws prescribe a strict penalty for this offence. To proscribe something is to ban it: The import of pornographic material was proscribed by law; The new nationalist party was proscribed.
The white saltire comes from the flag of Scotland. The St Patrick's Cross was, in fact, taken from the arms of the powerful Geraldine family. The red cross of St George is taken from the flag of England. Effective date: 1 January 1801.