Mounting modern NFS shares on IRIX is troublesome. NFSv4 is not supported, and UDP protocol is disabled on modern NFS implementations. So NFS filesystems on IRIX cannot be configured by the GUI anymore with recent file servers like TrueNAS.
But with the commandline below it's fairly simple:
truenas_fqdn:/mnt/tank/dataset /media/truenas nfs vers=3,proto=tcp,rw,bg,hard,intr,rsize=32768,wsize=32768 0 0