What is Irregular?

Legal Definition
That which is done contrary to the common rules of law; as, irregular process, which is that issued contrary to law and the common practice of the court. Vide Regular and. Irregular Process.
-- Bouviers Law Dictionary
Legal Definition
Not according to rule; improper or insufficient, by reason of departure from the prescribed course. As in irregular "Deposit," "Endorsement," "Process," and "Succession," sce those titles.
-- Black's Law Dictionary