[robespierre]

There are compatible cards from Central Data and Digi:

Software support for the Digi International ClassicBoard PCI family of
serial-port adapters is provided by the CDP package. (The CDP package
also supports the legacy Central Data PCI-4 and PCI-8 Serial Port
Adapters.) The CDP package is in the IRIX-standard inst(1M) format.

The CDP package consists of these subsystems:

cdp.sw.base base software required to use PCI products
cdp.man.relnotes these release notes

At present, support is provided for IRIX 6.5 only on IP27 (Origin2000
and Origin200), IP30 (OCTANE), and IP32 (O2) platforms.

from https://ftp1.digi.com/support/driver/9300356A.txt

There was also a SGI dual serial-port PCI card (SGI PCI Serial I/O (PCI SIO UFC), 030-1657-003), but apparently it only had drivers for IP35.