In a first approach to the spectral signatures of an irregular carbonaceous matter, we report a theoretical investigation of the infrared spectra of non-regular conjugated hydrocarbons. The series under consideration consists of three-cycle PAHs. Anthracene and phenanthrene are taken as references for regular PAHs and dibenzocyclobutadiene (biphenylene), dibenzocyclopentadienyl (fluorenyl), dibenzocycloheptatrienyl (heptafluorenyl) as models for irregular PAHs. These irregular conjugated hydrocarbons consist of four-, five- and seven- (instead of six) membered rings respectively sandwiched between two benzene entities while the regular ones are built with only six-membered rings. The IR spectra of the neutral, singly and doubly ionized species are compared. It is found that the non-regular compounds present the same general behaviour as regular ones at each level of ionization. From an astrophysical point of view, none of the differences is found big enough to be relevant for selective observation, which implies that all sorts of PAHs including irregular structures may be present and contribute anonymously to the UIR bands.