The Jan/Feb 1993 and May/Jun 94 Pipelines have detailed writeups on how
  to do this in IRIX 4.0.x and 5.x respectively. The Jul/Aug Pipeline
  has a correction to the latter article. If you like you can call the
  TAC and have them fax you the very latest version.

  It is also possible to add swapfiles within the filesystem structure.
  You can allocate space for swap in the filesystem by doing a 'mkfile'
  and adding this file to the system as swap space. See mkfile(1) for more
  information. The created file can be added by using a 'swap -a <file>'.
  If you intend this to be a permanent resource, make sure that you add
  an appropriate line to the /etc/fstab file. See the fstab(4) manpage
  for details.