As Catholics are we bound by the writings of Saints Faustina and Malachy?
No. Catholics are not bound by private revelations; only by public revelation, which ceased at the death of the last apostle. However, it should be noted that the Church does sometimes approve private revelations as worthy of human belief, leaving the decision of belief up to individual Catholics, as it has done with the Divine Mercy revelations believed to have been granted St. Faustina Kowalska. The prophecies attributed to St. Malachy, on the other hand, have not been approved.