Standard IRIX doesn't do very good with DHCP. That said;
1. Enable DHCP to get a lease for an IP address
isengard# /etc/chkconfig autoconfig_ipaddress on
2. Manually set default route
isengard# route add net default xxx.xxx.xxx.xxx
-and/or-
have route automatically added at boot
isengard# cat /etc/config/static-route.options
$ROUTE $QUIET add net default xxx.xxx.xxx.xxx
isengard#
3. Manually add DNS server addresses
isengard# echo "nameserver zzz.zzz.zzz.zzz" >> /etc/resolv.conf
isengard#
