That's strange. IRIN opens perfectly in my machine (iMacG5, MacOx10.4.6).
Before trying any fix, tell me:
Do you have Max/MSP installed, if so, is it version 4.5.7? for some strange reason MAX standalones are not a fully_self_contained app. If there’s a Cycling74 folder inside /library/application support, this folder will override the correspondent one inside the standalone app. If that one is older than the one from the standalone, it will cause problems.
Did you upgrade to MacOx10.4.6 via automatic updater (apple menu)?