IRIS is just a very simplistic missile warning system and only against common missiles since it only sees solar blind UV and filters out all other light spectrums to reduce clutter and requirement for a smarter image analyzer.
DAS and dozen other systems below use IR instead of UV because jet engines do not create UV so you can't see enemy fighters with our IRIS.
Also it is possible to make missiles that do not radiate in UV (like meteor and other ramjet AA missiles) and regular missiles radiate UV only when their engines are still active, so I think we made a huge mistake by choosing glass filtered UV, instead of AI filtered IR like NG-DAS and dozen others. But I am no expert. Correct me if I am wrong.
en.wikipedia.org