iRule to disable VIP with less then X members a pool

That's an interesting use case, and it's a little sad that this isn't just an option in the config. There's probably a few ways to do this, but here's what I've come up with using an external monitor. Add this monitor script to a pool that already has a good working monitor and make sure the Availability Requirement setting is set to All.

!/bin/sh
pidfile="/var/run/$MONITOR_NAME.$1..$2.pid"
 Send signal to the process group to kill our former self and any children
 as external monitors are run with SIGHUP blocked
if [ -f $pidfile ]
then
    kill -9 -`cat $pidfile` > /dev/null 2>&1
fi
echo "$$" > $pidfile

pool=local-pool
minup=2
virtual=access-test-vs

upmembers=`tmsh show /ltm pool $pool members |grep "Current Active Members" |awk -F" : " '{ print $2 }'`

if [ $upmembers -ge $minup ]
then
    rm -f $pidfile
    state=`tmsh show /ltm virtual $virtual |grep "State" |awk -F" : " '{ print $2 }'`
    if [ $state == "disabled" ]
    then
        logger -p local0.info -t MONITOR-ALERT "Pool $pool Monitor UP - enabling virtual $virtual"
        tmsh modify /ltm virtual $virtual enabled
    fi
    echo "up"
else
    rm -f $pidfile
    state=`tmsh show /ltm virtual $virtual |grep "State" |awk -F" : " '{ print $2 }'`
    if [ $state == "enabled" ]
    then
        logger -p local0.info -t MONITOR-ALERT "Pool $pool Monitor DOWN - disabling virtual $virtual"
        tmsh modify /ltm virtual $virtual disabled
    fi    
    echo "up"
fi

The script itself always reports "up" to the monitor because its job is simply to monitor the number of available members, compare that to the "minup" value, and then enable or disable the VIP (if it's already disabled/enabled respectively). Also notice that I hard-coded the pool, minup and virtual values:

pool=local-pool
minup=2
virtual=access-test-vs

You could very easily turn those into arguments in the external monitor config:

ltm monitor external test_nodesup_monitor {
    args "local-pool 2 access-test-vs"
}

That way the script could use $3 (the pool), $4 (minup), and $5 (virtual server) instead of the hard-coded values. You'll still need to create a separate external monitor config for each VIP (and pool) that you want to work with.