Fortunately, it is super easy to get a list of down devices in LibreNMS. All we need to do is a dump of the devices from MySQL.
You can use the following command to print which devices are down. Should be the same devices that show down on your dashboard.
mysql -u librenms -p librenms -e 'use librenms ; select hostname,sysName,status from devices where status=0'
Tip: You can change 0 to 1 to see all up devices.