The IRQ should not pop up as an issue (famous last words), but what I have seen is Windows reassigning the "default" as it sees fit unless one is specifically assigned as default by the user (at least in Win7). The way to do this is to have both devices online and visible to Windows; and even if the correct one is default (Realtek HD) in Windows Sound Card properties, assign the other (audio interface), and then assign the Realtek HD again... that way the user
explicitly chose "Realtek HD" as the default and it should stick from that point going forward.
After doing this, Windows should never "take control" of the audio interface on you, just make it available for use.