RMC,
Had the same the other day in the aircraft. We were sans APU and the GPU just died. The horn sounds immediately. It isn't anything to do with the APU. I often hear that alarm when doing a walk around and an adjacent aircraft is doing a fire test and is a completely different noise from the wheel well.
The faint beeping you can hear from the flight deck is am external warning to the ground engineer that the aircraft is on battery power and that the battery is being depleted. I don't even think there is any specific relation to the IRS.