What was found
UNABLE RNP ALERT AT CRUISE. ATC ADVISED OF OUR STATUS. ATC LATER INFORMED US WE WERE 40 MILES WEST OFF COURSE. DURING CREW DEBRIEF THE POS OF RNP ACTUAL WAS SHOWING 1.00/6.00 NM. IRS/GNS POS UPON ARRIVAL SHOWED ALL THREE IRUS GREATLY MISCOMPARED LAT/LONG TO EACH OTHER AND TO GNS. DOWNPOWERED IRU'S, RESET FMC CIRCUIT BREAKERS IAW MM 34-63-09, REALLIGNED IRU'S IAW MM 34-43-00-5 WITH ALL IRUS AND GNS'S MATCHING PERFECTLY. NO UNABLE GNS MESSAGE PRESENT. AFTER RESET RNP/ACTUAL WAS READING 1.00/0.02NM. ACFT OK FOR SERVICRE. ACCOMPLISHED R-T-S OF IRU'S PER MM 34-43-00-5, ALL CHECKED NORMAL, OK FOR SERVICE.