To Enable rtirq
To enable the rtirq script (package name rtirq-init), the kernel needs to be booted with the threadirqs argument. For this to work, the kernel needs to have been built with CONFIG_IRQ_FORCED_THREADING=y (which is done on both linux-lowlatency and linux-generic).
To enable threadirqs on a kernel by default, it can be built with the config options CONFIG_CMDLINE_BOOL=y and CONFIG_CMDLINE="threadirqs".
To boot with the threadirqs, an easy way to add it to the boot parameters for GRUB is to add the argument in /etc/default/grub and edit the line GRUB_CMDLINE_LINUX="" to read GRUB_CMDLINE_LINUX="threadirqs".
If wanting to try a newer version of the script, or updating the current one, the files of interest are /etc/default/rtirq (configuration), and /etc/init.d/rtirq (the actual script).