What was found
L IRS NOT DISPLAYING ANY INFORMATION IN FLIGHT AT FL340. NOTE- POWER INTERRUPTION ON THE GROUND DUE TO AFTER START FLOW BEING DONE INCORRECTLY (APU OFF BEFORE ESTABLISHING ELECTRIC- GEN). RE-RAN CHECKLISTS, CHECKED C/B, AND DID NOT NOTE ANY ANOMALIES. GROUNDED - YES. AIR TURNBACK - FLT CREW TURNED BOTH IRS SYSTEMS OFF AFTER ARRIVAL AT GATE. L/H IRS SHOWED NO FAULTS INFLT OR CURRENT STATUS. REMOVED AND REPLACED L/H ADIRU PRECAUTIONARY REF B737-900 AMM 34-21-01-400-801 AND ALL TESTS PASSED REF B737-900 FIM 22-11 TASK 802 LRU REPLACEMENT TEST FOR ADIRU.