FATIGUED FLT CREW OF B737-300 EXPERIENCES DUAL IRU FAILURE IN IMC CONDITIONS. LOSS OF ALL BUT STANDBY INSTRUMENTS DIRECTS NO GYRO VECTORS BACK TO DEP ARPT AND A SAFE LNDG.

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FLT FROM ZZZ. MY DAY CONSISTED OF A CITY-CITY COMMUTE; FOLLOWED BY A CITY TO ZZZ FLT # WITH A SCHEDULED 3 HR 55 MIN ARPT WAIT FOR FLT # FROM ZZZ. THE INBOUND FLT WAS LATE DUE TO A PLANE CHANGE UP LINE AND MY ARPT APPRECIATION TURNED INTO 5 HRS 42 MINS (OF WHICH THE LAST 23 WERE GREETING THE INBOUND FLT AND PREFLT PHASE). FO WAS ABOARD HAVING FLOWN THE PREVIOUS 3 LEGS. HE TOO HAD A LONG DAY. I TALKED TO HIM IN THE PLT LOUNGE PRIOR TO MY DEADHEAD AND HIS STARTING THE TRIP. DURING THE CREW BRIEF I ASKED HIM 'WHOSE LEG?' HIS COMMENT TO ME WAS 'YOURS.' TKOF FROM RWY 36R UNEVENTFUL. SWITCHED DEP; ENGAGED AUTOPLT; AND CLRED TO 12000 FT. DEP THEN GAVE A 'TURN L TO 350 DEGS.' I DIALED '350' INTO THE COURSE WINDOW AND ENGAGED HDG SELECT. THE ACFT BEGAN A L TURN WITHOUT STOPPING AT WHAT I THOUGHT WAS 350 DEGS. IT WAS THIS TIME I NOTICED THE HSI COMPASS CARD WAS REVERSED (S WAS N). GIVING A CLOSER LOOK AT ALL INSTS INDICATED THE ADI WAS NOT FUNCTIONING; NOR WAS THE IVSI (ALL HAD FROZEN STRAIGHT AND LEVEL WITH BOTH COMPASS CARDS REVERSED). AIRSPD AND ALTIMETER WERE FUNCTIONING. THE IRS CTL PANEL WAS BLANK; RECALL CLR. I DISENGAGED HDG SELECT; TRANSITIONED TO THE STANDBY GYRO AND WET COMPASS. I DIRECTED THE FO TO TELL ATC OUR NAV SYS WAS INOP. ATC THEN ASKED WHAT WAS WORKING. THE VOR WAS FUNCTIONING BUT THE NEEDLE WAS SLAVED TO THE RDMI WHICH HAD ALSO FAILED. I TOLD FO TO GET THE QRH OUT FOR IRS FAILURE AND START WITH THE PROCS. I TOOK OVER ATC COM. IMC STILL EXISTED AND I ASKED FOR A CLB TO VMC. ALL WE HAD WAS THE AIRSPD INDICATOR; ALTIMETER STANDBY GYRO; AND WET COMPASS. WE CONTINUED THE CLB TO FL230 AND GAINED VMC. LEVELED OFF AND BEGAN THE QRH CHKLIST. NO QRH PROC MET WHAT WE HAD BUT WE DID GO WITH THE IRS FAILURE/ATTITUDE DISPLAY DOES NOT RECOVER CHKLIST. THIS IS WHEN WE SWITCHED TO ATT AND LOST THE AUTOPLT; IMC AGAIN AND I REQUESTED A HIGHER ALT. VMC AGAIN AT FL260. NOTHING WE DID SEEMED TO WORK. FO DID GET THE HDG INPUT IN THE CDU AND THE RMI SLAVED TO A PROPER DIRECTIONAL HDG. I'D HAD ENOUGH BY NOW AND TOLD ZZZ1 CTR TO GIVE NO-GYRO VECTORS BACK TO ZZZ. FO BRIEFED THE FLT ATTENDANTS AND PAX ON OUR INTENT TO RETURN TO ZZZ. I TOLD FO TO BACK ME UP ON FLYING; CHKLISTS; AND COM. (THIS SINCE I WAS GETTING THE START TURN; STOP TURN COMMANDS.) MY PRIMARY INSTS WERE THE ALTIMETER; AIRSPD AND THE STANDBY GYRO. THE DSCNT WAS UNEVENTFUL WITH A NO-GYRO APCH. MAINT WAS STANDING BY AT THE GATE. 'DUAL IRU FAILURE INFLT' WAS THE LOGBOOK ENTRY. DISPATCH ASSIGNED ANOTHER ACFT WHICH WE FLEW UNEVENTFULLY TO OUR DEST. I DON'T KNOW WHAT COULD BE DONE TO PREVENT THIS NON-NORMAL FROM OCCURRING AGAIN BUT FOR MY PART IN THIS; I FEEL I DID NOT COMMUNICATE PROPERLY WITH MY FO AS FAR AS HOW TO CONDUCT THE QRH ACTIVITIES. I DID TELL HIM TO GO TO THE IRS FAILURE CHKLIST AND GO THROUGH THE PROCS. HE DID THIS. I FELT I WAS TASK SATURATED WITH FLYING THE AIRPLANE AND THIS PREVENTED ME WITH HELPING HIM CONDUCT THE APPLICABLE STEPS. I THINK WE (I KNOW I WAS) WERE CONFUSED BECAUSE WHAT WE WERE SEEING DID NOT MATCH ANY OF THE QRH CONSIDERATIONS. (THE 'TAKE APPROPRIATE ACTION' PHASE WAS MORE OF AN AD-LIB AND SEE WHAT WORKS PHASE.) WHEN ATT WAS SELECTED AND THE AUTOPLT KICKED OFF; I FELT VERTIGO ONSET AND MY ATTN WAS MORE ON THE STANDBY GYRO THAN ON WHAT FO WAS DOING. ONCE A HDG WAS INPUT INTO THE CDU; I WAS ABLE TO RE-ENGAGE THE AUTOPLT. THIS LASTED FOR A LIMITED TIME; THEN KICKED OFF AGAIN. ANYWAY; WE MADE IT DOWN SAFELY. IT WAS AN EYE OPENER FOR ME AND HOPEFULLY DURING MY NEXT NON-NORMAL; I WILL DIRECT THE QRH PROCESS INSTEAD OF BEING OVERCOME BY EVENTS THAT ARE NOT EXPECTED.