Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
1. irrīsus, a, um, Part., from irrideo.
2. irrīsus (inr-), ūs, m. [irrideo], a scoffing, mocking, mockery, derision: irrisu coarguere aliquid, Plin. 28, 8, 29, § 114: irrisum pueri sperans, Tac. A. 13, 15: irrisui esse, to be a laughing-stock, Caes. B. C. 2, 15: hostibus irrisui fuit, Tac. A. 14, 39; id. H. 1, 7: scripsisse eos non sine irrisu generis humani arbitror, Plin. 37, 9, 40, § 124: irrisui haberi, to be made a laughing-stock of, be made game of, App. M. 5, p. 172: ab irrisu, out of mockery, Liv. 7, 10.