The secret (at least in this case) is to byte-swap the tape before
passing it to tar:

  # dd if=/dev/rmt/0m conv=swab | tar -xvf -

Byte swapping is believed to only be necessary if the device which
created the tape was a swapping one (swapping tape devices are the
default on IRIX 4, but not in IRIX 5).  One can use /dev/nrtapens on
either system to produce tapes which are not byte-swapped.
If the SGI is running Irix 5.0x and above, a large (512k) block
size is used:

  # dd if=/dev/rmt/0m ibs=512k obs=10k| tar -xvf -