I spun up a new instance of Ubuntu and it installed fine.

Thanks for your help.

Trying that now. Thanks for your help, I'm hoping this works.

so, I run:

sync; echo 1 > /proc/sys/vm/drop_caches

Then:

                       total        used        free      shared  buff/cache   available
Mem:           3936         301        3414           6            220                3401

Then:

/tmp/iriskit/IRIS_Community-2021.1.0.215.0-lnxubuntux64/irisinstall

Still results in:

Starting installation
Starting up InterSystems IRIS for loading...
../bin/irisinstall -s . -B -c c -C /usr/iris/iris.cpf*IRIS -W 1 -g2 
Starting Control Process
Unable to allocate 242 MB shared memory...
Unable to allocate 232 MB shared memory...
Configuring minimum system...
Unable to allocate 139 MB shared memory...
Unable to allocate shared memory minimum of 139MB (errno=22)
Startup aborted.
Startup error. See messages.log for more information.
Call InterSystems Technical Support if you need assistance.
IRIS startup returned 1.
InterSystems IRIS failed to start.
Check /usr/iris/mgr/messages.log for more details.
Call InterSystems Technical Support if you need assistance.

** Installation aborted **

 Private  +   Shared  =  RAM used       Program

100.0 KiB + 130.0 KiB = 230.0 KiB       none
216.0 KiB + 252.0 KiB = 468.0 KiB       lightdm-greeter
208.0 KiB + 286.0 KiB = 494.0 KiB       atd
224.0 KiB + 280.0 KiB = 504.0 KiB       acpid
324.0 KiB + 492.0 KiB = 816.0 KiB       cron
308.0 KiB + 658.0 KiB = 966.0 KiB       agetty (2)
436.0 KiB + 606.0 KiB =   1.0 MiB       irqbalance
668.0 KiB +   1.2 MiB =   1.9 MiB       avahi-daemon (2)
608.0 KiB +   1.4 MiB =   2.0 MiB       master
948.0 KiB +   1.3 MiB =   2.2 MiB       at-spi2-registryd
516.0 KiB +   1.8 MiB =   2.3 MiB       pickup
952.0 KiB +   1.4 MiB =   2.3 MiB       kerneloops (2)
  1.0 MiB +   1.4 MiB =   2.4 MiB       at-spi-bus-launcher
588.0 KiB +   1.8 MiB =   2.4 MiB       qmgr
  1.1 MiB +   1.4 MiB =   2.6 MiB       gvfsd-fuse
  1.3 MiB +   1.5 MiB =   2.8 MiB       ntpd
  1.4 MiB +   1.5 MiB =   3.0 MiB       monit
  1.2 MiB +   1.9 MiB =   3.0 MiB       gvfsd
  1.2 MiB +   1.9 MiB =   3.1 MiB       accounts-daemon
  1.3 MiB +   2.5 MiB =   3.8 MiB       systemd-logind
  1.7 MiB +   2.4 MiB =   4.1 MiB       whoopsie
  1.9 MiB +   2.5 MiB =   4.4 MiB       upowerd
  2.2 MiB +   2.3 MiB =   4.6 MiB       rsyslogd
  2.2 MiB +   2.9 MiB =   5.1 MiB       cupsd
  2.0 MiB +   3.4 MiB =   5.4 MiB       cups-browsed
  2.7 MiB +   2.9 MiB =   5.6 MiB       bash
  2.1 MiB +   3.8 MiB =   5.9 MiB       tlsmgr
  2.4 MiB +   3.5 MiB =   5.9 MiB       udisksd
  2.7 MiB +   3.3 MiB =   6.0 MiB       systemd-networkd
  3.0 MiB +   3.1 MiB =   6.1 MiB       redis-server
  2.9 MiB +   3.7 MiB =   6.6 MiB       polkitd
  2.6 MiB +   4.0 MiB =   6.6 MiB       gnome-keyring-daemon
  2.6 MiB +   4.2 MiB =   6.8 MiB       dbus-daemon (4)
  1.7 MiB +   5.3 MiB =   7.0 MiB       (sd-pam)
  2.7 MiB +   5.2 MiB =   7.9 MiB       lightdm (3)
  4.0 MiB +   4.2 MiB =   8.2 MiB       systemd-udevd
  2.6 MiB +   6.6 MiB =   9.2 MiB       sshd (2)
  4.4 MiB +   5.5 MiB =   9.8 MiB       systemd-resolved
  4.1 MiB +   7.7 MiB =  11.7 MiB       packagekitd
  6.2 MiB +   6.3 MiB =  12.5 MiB       droplet-agent
  3.3 MiB +  11.5 MiB =  14.8 MiB       systemd (2)
  5.0 MiB +  10.3 MiB =  15.3 MiB       nginx (6)
  8.1 MiB +  10.1 MiB =  18.2 MiB       networkd-dispat
  8.5 MiB +  11.9 MiB =  20.4 MiB       unattended-upgr
  7.1 MiB +  15.1 MiB =  22.2 MiB       systemd-journald
 12.7 MiB +  14.1 MiB =  26.8 MiB       multipathd
 11.9 MiB +  15.9 MiB =  27.8 MiB       f2b/server
 15.0 MiB +  15.0 MiB =  30.0 MiB       do-agent
 31.4 MiB +  31.5 MiB =  62.9 MiB       snapd
 18.2 MiB +  46.8 MiB =  64.9 MiB       Xorg
 21.9 MiB +  56.2 MiB =  78.1 MiB       php-fpm8.0 (22)
 39.0 MiB +  69.5 MiB = 108.5 MiB       lightdm-gtk-greeter
---------------------------------------------------------------------
                                         667.6 MiB
=================================

                       total        used        free      shared  buff/cache   available
Mem:           3936         300         178           6              3457        3365

Hmmm. 3365 available but only 178 free?

The VM has 4GB RAM. I run the Iris CE docker container on a Docker VM with 2GB or RAM so I would have thought 4GB of RAM would be adequate on the stand-alone VM.