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 **