So far the only fossil that has been found was an 80 centimeter long fossil skull in the Romualdo Member, a layer member of the Brazilian Santana formation. This skull strongly resembles the skull of Spinosaurus. The genus is often regarded today as identical (synonymous) with Angaturmata, which lived in the same time and the same place as Irritator.
Irritator was first scientifically described in 1996 by paleontologists Martill, Cruikshank, Frey, Small and Clarke. Its only known fossil, an 80 cm skull discovered in eastern Brazil, was badly obscured by plaster which was added by the commercial fossil-collecting fossil-poachers who illegally sold it (the trade of fossils is prohibited by law in Brazil), in hopes of making the fossil look more complete and valuable. The buyers were not aware of the modifications to the illegally collected specimen, and it required them a great deal of work to reconstruct the original features — hence the name.