It all depends on which services you expose to the internet.
A firewalled IRIX machine with no open ports is going to be impossible(*) to crack from the internet, but you must realize that IRIX 6.5 was introduced in 1998, and even IRIX 6.5.30 is some 7 years old now. This is ancient history in internet time.
IRIX 6.5.30 installation comes with OpenSSL 0.9.7e and OpenSSH 3.0p1. You could install IRIX Patch 7246 (OpenSSH security patch, 14-Apr-2011), and IRIX Patch 7217 (OpenSSL security patch, 09-Sep-2008), but a quick look
HERE will show you how many OpenSSL vulnerabilities were reported since (and that doesn't include OpenSSH vulnerabilities).
Last patches to 'named' were in November 2009. I don't run bind on my IRIX systems, but it's probably some obsolete 8.x version. Again, many, many issues.
Ditto for just about any other service (apache, ftpd, ...)
While the system is vulnerable, there's most likely nobody left that bothers to write exploit code for IRIX system anymore. This is called security through obscurity
So, my advise: if you want to run an IRIX system as an internet server, compile your server software from (current) source code, and firewall everything else. I believe this is what Pete did as well, back when he ran Nekochan on an Origin 350.
(*) Unless there's a bug in the firewall.
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