The following are irregular for receiving orders: 1/ a person who labors under some form of amentia or other psychic illness due to which, after experts have been consulted, he is judged unqualified to fulfill the ministry properly; 2/ a person who has committed the delict of apostasy, heresy, or schism; 3/ a person who has attempted marriage, even only civilly, while either impeded personally from entering marriage by a matrimonial bond, sacred orders, or a public perpetual vow of chastity, or with a woman bound by a valid marriage or restricted by the same type of vow; 4/ a person who has committed voluntary homicide or procured a completed abortion and all those who positively cooperated in either; 5/ a person who has mutilated himself or another gravely and maliciously or who has attempted suicide; 6/ a person who has placed an act of orders reserved to those in the order of episcopate or presbyterate while either lacking that order or prohibited from its exercise by some declared or imposed canonical penalty.
Source: Code of Canon Law (Vatican.va)