If the script isn't to long, you can just copy and paste the commands in the script into /etc/rc.d/rc.local. If the script is long, just put this in rc.local:
cd <path to script directory>
rc.local is the last init script to run and it runs with root privilege like all init scripts. It's generally used to quickly add something to the init sequence and is included in most linux distros but not debian based distros.