The Dallas DS1397 isn't manufactured anymore. You may be able to buy one but who knows what the state of it's internal battery is. This device is available as chip (DS1395/DS1395S) or stand alone module with embedded lithium battery and crystal (DS1397). A quick look at the datasheet makes it obvious that the battery contacts must be hiding behind the 'NC' contacts pin 18 and 22.

My initial plan was to desolder the DS1397 from the IO4 first, and then open the package. Unfortunately the IO4 is a very thick, heavy multilayer PCB and even with a profesional desoldering station I wasn't able to remove it from the PCB. So I had to open the package while the DS1397 was still installed on the IO4 board. Fortunately this was much easier than I expected, but you have to temporarily remove the support bar from the IO4 because it isn't possible to access the right side of the Dallas chip otherwise.

I used an SMD version of the socket because it's flat underside is easily glued in the empty space next to the Dallas chip. Be careful that the pads of the CR2032 socket do not touch any of the nearby via's in the PCB. Connect the socket to the exposed V_bat and B_gnd pins, and finish it off with some hot glue.

The empty or invalid configuration data in the NVRAM will make an Onyx default to a console on the serial port rather than the graphics output. A 'resetenv' from the PROM prompt will restore default NVRAM values. You may have to change a few values if your system disk is on SCSI channel 1, and (for an Onyx) set the console variable to 'g'. The first time you boot into IRIX it will complain about a preposterous time in the year 2072, but once the time is set everything is good as new!