B737 flight crew reported while in cruise the autopilot disconnected; and the First Officer lost all primary flight information.

Narrative

At cruise at FL310; north of ZZZZZ enroute to ZZZZ; the Autopilot disconnect horn sounded and the Autopilot disconnected. The First Officer (FO) stated he'd lost all primary flight information; including attitude; airspeed; altitude; heading; and position. His displays were blank except for numerous failure flags. I put my hands on the flight controls and thrust levers and hand flew the aircraft. My flight instruments and flight director were working. I tried to turn the Autopilot back on but it would not engage. At that point; I attempted to diagnose the situation and determine what checklist to run. Other than the OFF flags on the First Officer's (FO's) displays there were no fault indications on the primary display units. I asked the FO to check the overhead panel and he discovered the Inertia Reference System (IRS) 2 FAULT light and the Electronic Engine Control (EEC) ALTERNATE lights were illuminated. I asked the FO to get the QRH for IRS FAULT and I would fly the aircraft.At this point I realized we were approaching the ZZZZ FIR and I did not want to cross the FIR boundary with this malfunction. I told the FO; he suggested asking for a hold and I concurred. We started a hold at ZZZZZ and the FO ran the QRH for IRS FAULT. The QRH had us align the affected IRS in ATT mode to recover the attitude display on the FO side. At this point we were heading back to the north and we asked ATC if we could continue on our current heading to accomplish the alignment. This recovered the attitude display for the FO but he still had no air data or position. The QRH also directed us to not use the Autopilot. I asked the FO to look for any popped circuit breakers. He didn't see any but said he'd heard a loud click just before the Autopilot disconnected. I asked the FO to tell ATC we needed to return to ZZZ; and ATC gave us a clearance direct to ZZZZZ1 for the ZZZZZ1 arrival into ZZZ.The purser then called me to tell me that the passengers realized we had turned around and wanted to know why. One passenger in particular was very upset. I explained to the purser what was going on and then made an announcement to the passengers; telling them we had a malfunction that did not allow us to continue our flight to ZZZZ and that we would be returning to ZZZ. The workload was very high. I normally expect in non-normal situations to give the aircraft to the FO while I run the checklist and manage the situation. In this case; I needed to fly because only I had flight instruments; but I couldn't use the Autopilot and still needed to manage the situation as the Captain. I told the FO we would [request priority handling] and return to ZZZ and plan the ILS to Runway XX. The FO wanted to recover his airspeed and altitude displays. He suggested running the airspeed unreliable checklist and I agreed; but this had no additional help. We also tried putting the IRS selector to BOTH ON 1; but this had no effect. I asked the FO to send a message to Dispatch and Maintenance Control telling them we'd lost IRS 2; all FO instrumentation; and the Autopilot; and that we'd be diverting to ZZZ and [requesting priority handling].At this point; I asked the FO if he could fly briefly while I coordinated with the flight attendants (FAs) and ATC and set up the arrival. The FO had to fly using his attitude display and the standby flight display; but I thought this was a reasonable course of action at cruise in VMC as it allowed me to ensure we'd done the correct procedures; ensure the proper coordination with Dispatch; Maintenance Control and station had been done; and brief the flight attendants and passengers; and generally to make sure my SA was where it needed to be.The FO noted we were likely non-RVSM compliant; and I so advised ATC; who gave us a descent to FL280. I briefed the purser. I did not have them prepare the cabin for evacuation. I made another announcement to the passengers explaining our situation and telling them our customer service team would be onhand to help once we landed at ZZZ. I also reviewed the QRH to ensure we hadn't missed anything and noted we needed to run the EEC alternate mode QRH; which I then ran. I coordinated again with Dispatch and Maintenance Control; and ensured the relevant information was passed and they were ready for the diversion.We had run the IRS FAULT; Airspeed Unreliable; and EEC Alternate Mode checklists. We had recovered the FO's attitude display; but were unable to recover the FO's position; altitude; flight director; or airspeed; or the autopilot. I suspected a complete failure of the #2 ADIRU.I set up and briefed the arrival. I told the FO to not let ATC increase our workload; and to be prepared for emergency vehicles to be present after landing; and to make a passenger announcement if this was the case. I also briefed that while we suspected a complete #2 ADIRU failure; we weren't completely sure and to be vigilant to any other anomalies during the approach and landing. We [requested priority handling] with ATC and requested the ILS to Runway XX; but ATC said it was unavailable and to expect XYL.I flew the arrival and ILS to a normal landing on Runway XYL; and we taxied to the gate uneventfully. I gathered the entire crew after the passengers deplaned to debrief everyone's observations and answer any questions. One of the flight attendants was unhappy with the communication from the flight deck. I did make two passenger PAs explaining the situation and I briefed the purser. I think two factors contributed to the flight attendants' perception. First; the decision to turn around before entering the ZZZZ FIR; which I think was a sound decision; caused passengers to become aware that something not normal might be occurring. Second; at that point we had degraded instrumentation and no Autopilot; and were just starting to try to recover what systems we could. While more timely communication might have been preferable; I still needed to prioritize our tasks on the flight deck in a high workload situation.