Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
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.