Usually the irregularity consists in a word having two different stems.
a. Both stems have a common nominative singular: σκότος
N. Many compound proper names in -ης (especially names of foreigners) have forms of the 1 and 3 decl., as Τισσαφέρνης, -νους, -νῃ and -νει. So Θεοκρί_νη (voc.) in Demosth., Λεωνίδην and Λεωνίδεα in Hdt.
b. Certain cases are formed from another stem than that of the nom. singular: ὁ ὄνειρο-ς
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].