From the ALSA wiki
The rtirq script is a bash script written by Rui Nuno Capela which works in conjunction with the realtime kernel patch by Ingo Molnar. The script takes advantage of the fact that realtime kernels use threads for IRQs management, and as such these threads (like any other thread running on your system) can be given maximum priority in an effort to minimize the latency of audio peripherals.
The script identifies the audio devices present on the machine and raises the priority of the threads that handle the IRQs of such devices. The rtirq script requires that the 'schedutils' package is correctly installed on the system, and works by simple invocation. For instance:
$ sh rtirq.sh rtirq.sh: start [rtc] irq=8 pid=247 prio=90: OK. rtirq.sh: start [snd] irq=5 pid=1045 prio=85: OK. rtirq.sh: start [uhci_hcd] irq=10 pid=747 prio=80: OK. rtirq.sh: start [ehci_hcd] irq=11 pid=289 prio=80: OK. rtirq.sh: start [i8042] irq=1 pid=297 prio=75: OK. rtirq.sh: start [i8042] irq=12 pid=296 prio=74: OK.
The script has identified an external USB card and has hence raised the priority of the threads managing the USB bus (uhci_hcd and ehci_hcd).