I've found older posts instructing adding the line "/usr/sbin/irqbalance" to the file /etc/rc.local after installing irqbalance. I also noticed after installation that there is an irqbalance file in /etc/config with an enable option in it. I've set the enable option to "1" from "0" and rebooted.
So I'm feeling pretty good that I have this thing installed on all my multi-core CPU routers now.
Do I have to modify both rc.local and the irqbalance file in /etc/config, or is setting enable to "1" in /etc/config/irqbalance all that is needed to activate irqbalance now?