A B737 FLT CREW DSNDING INTO DEN ON THE LARKS ARR EXPERIENCES AN IRS FAILURE THAT CAUSES AN AUTOPLT HARD OVER AND PITCH DOWN JUST AFTER THEY GET A TCAS.

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ARR IN DEN INITIALLY WAS POWDR 5 AND WAS CHANGED TO LARKS 5. WE RECEIVED CLRNC TO CROSS LARKS 250 KTS/17000 FT TOO LATE. WE MADE 17000 FT BUT THE SPD WAS 290 KTS. NEXT CLRNC WAS DSND TO 14000 FT SPD 210 KTS. A FEW SECONDS LATER ATC SAID STOP YOUR DSCNT; STOP YOUR DSCNT. WE STOPPED AT 16000 FT; NEXT WAS TURN L HDG 350 DEGS. I START TURNING AND TCAS WENT OFF. I FOLLOW THE TCAS AND TURN R 20 DEGS TO AVOID TFC NEXT TO US AT 10 O'CLOCK POS. TFC WAS AT 16500 FT DSNDING; RADIO FREQ WAS SATURATED; EVERYBODY STEPPING ON EACH OTHER; WE DID NOT HAVE A CHANCE TO TELL ATC WHAT HAPPENED. WE GOT THE FREQ CHANGE AND AT THIS TIME L IRS FAILED WITH AUTOPLT ON. WE EXPERIENCED A HARD OVER AND AGGRESSIVE NOSE DIVE. I DISCONNECTED AUTOPLT AND AUTOTHROTTLE AND RECOVERED THE AIRPLANE. I LOST EADI AND EHSI; I TOLD FO COULD YOU XFER YOUR PICTURE TO MY SIDE; HE SAID I SURE WOULD LIKE TO HELP BUT I JUST LOST MINE. AT THIS MOMENT STANDBY INSTS WERE THE ONLY THING I COULD USE. APCH AND LNDG WERE UNEVENTFUL. AFTER WE PARKED WE CALLED ATC. IT WAS A MUTUAL DISCUSSION; WE AGREED THAT A LOT HAPPENED IN JUST A FEW MINS; THE ONLY THING WAS THE OUTCOME; NO DAMAGE; NOBODY GOT HURT.