A very close practicing (but questioning) Catholic friend of mine from Ireland is having some trouble with this issue: St. Brighid, the female patron st. of Ireland, was apparently decanonized in 1960. Would the original declaration of her sainthood have been considered infallible at the time? How would you answer this as a question of infallibility?
Saint canonization is indeed infallible. That being said, a Saint may be removed from liturgical calendars for a number of reasons but such action should never be understood to mean that he or she is no longer recognized as a Saint.
St. Brigid of Ireland was never de-canonized. For more information on her see: St. Brigid