Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
sŭb-īrascor, ātus, 3, v. dep. n., to be somewhat angry (Ciceronian): interdum soleo subirasci, Cic. Fin. 2, 4, 12: brevitati litterarum, id. Fam. 11, 24, 1: in Epirum quod me non invitas, subirascor, id. Att. 9, 7, 7.
Hence, sŭbīrātus, a, um, P. a., somewhat angry: tibi, Cic. de Or. 1, 16, 72; id. Fam. 3, 9, 1: anus subiratior, App. M. 4, p. 154 (dub. al. iratior).