A-320 CREW HAD ACFT IRU'S MALFUNCTION PROVIDING FALSE NAVIGATION DATA TO THE ACFT.

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PRR FLT #1 INERTIAL REFERENCE HAD A DELAY MAINT STATUS MSG. MAINT MEL'D IT AS AN INDICATION PROB. TOOK OFF AND DURING CLB #1 MCDU HAD A CHK GROSS WT MSG. NO PROB FOUND IN CLB NOTICED CAPT'S HDG/TRK WAS 40 DEGREES OFF OF FO'S AND HAD A HEADWIND OF 160 KTS. FO WAS 10 KT TAILWIND. SWITCHED CAPT'S ATT HDG SWITCH TO #3 IR ALL NORMAL. OVER LAS DID A NAV POS ACCURACY TEST. NORMAL EXCEPT #1 IR WAS 13.0 MILES OFF/DIFF. NOTIFIED MAINT. THEY INFORMED US THAT THEY HAD RPTS FROM ACFT'S CFDS THAT #1 IR HAD MINOR INTERNAL PROBS. OVER RWF; ATC ASKED IF ON A HDG ASSIGNED (WE WERE NOT). ATC SAID WE WERE 10.13 MILES OFF AND GOING FURTHER. CAPT AND FO NAV DISPLAYS SHOWED ON COURSE. TUNED TO RWF VOR AND SAW THAT IN FACT WE WERE OFF COURSE. RIGHT BEFORE ATC CALLED US; WE GOT A 'NAV ACCURACY DOWN GRADE.' WE ASKED FOR RADAR VECTORS TO ARPT. DURING VECTORS I CHKED IR POS. #1 IR SHOWED 69.8 MILES DIFF. AFTER LNDG AND TAXI TO GATE; WE FINALLY GOT A #1 IR FAULT MSG. HAD WE BEEN IN MOUNTAINOUS NON RADAR ENVIRONMENT. THIS COULD HAVE BEEN A POSSIBLE TRAGIC EVENT. I FEEL MAINT DISPATCHED US WITH AN INCORRECT MEL. I ALSO FEEL THAT THE ACFT'S SENSING/WARNING/DETECTION SYS NEEDS TO BE A LOT MORE SENSITIVE TO WARN OF ERRORS LIKE THIS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE IRU'S IN THE ACFT IN ESSENCE USES A TRIPLE MIX POSITION TO SEND TO THE FMC'S. THIS IS THEN UPDATED IN THE FMC BY THE ACFT NAV SYS. APPARENTLY THE SINGLE IRU ERROR WAS ENOUGH TO THROW OFF THE TRIPLE MIX POS; YET NOT FAR ENOUGH IN ERROR TO GET VOTED OFF LINE OR TRIGGER A WARNING. THE MOVING MAP DOES NOT HAVE THE CAPABILITY TO SHOW RAW DATA IRU POS ON THE MAP. THE ACR ONLY REQUIRES THE PLT MONITOR RAW DATA WHEN ON APCH. THE ACR CHIEF PLT INDICATED THAT THE ACR MAY CHANGE SOP TO REQUIRE RAW DATA MONITORING BEGINNING AT THE TOP OF DSCNT. ACR X WAS DSCNDING ON THE SKEETER 2 ARR FOR LNDG AT MSP. UPON XING RWF (REDWOOD FALLS). MINNEAPOLIS CENTER NOTIFIED US THAT WE WERE OVER 10 MILES OFF COURSE. BOTH THE CAPT AND FO FLIGHT DISPLAYS WERE INDICATING THE ACFT WAS ON COURSE ON THE ARR. IN ADDITION; NO WARNINGS OR INDICATIONS WERE PRESENT FOR NAVIGATION FAULTS. THE NUMBER 1 ADIRU HAD A CLASS II DEFERRAL BUT THIS WAS NOT SUPPOSED TO AFFECT IT'S OR THE OTHER 2 ADIRU UNITS (INERTIAL; RADIAL NAVIGATION SYSTEM) IN ANYWAY AND THERE ARE NO SPECIAL PROCS FOR OPERATING THIS CONDITION. THE FLT CONTINUED ON VECTORS AND A VISUAL APCH AT THE ARPT. AFTER THE ACFT TAXIIED OFF THE LNDG RWY; NUMEROUS NAVIGATION AND RELATED WARNINGS APPEARED ON THE ECAM. THE ERROR RECORDED AT THIS POINT WAS NEARLY 60 MI FROM WHERE THE ACFT WAS.