I noticed one interesting thing with raspbmc today, It seems that the raspberry pi may not be powerfull enough to keep up with subtitles. I was watching a 720p video with Japanese audio and english subs, and it would do one or two sentences, and then if another person starts talking too quickly after it would not display any subs at all until the next break in speech.
I made sure it wasn't an issue with the file itself, and it played perfectly fine with the subs working as normal on my PC.
It could be because it seems to constantly use around 80% CPU even when just left idle on the menu (so it looks like running Irssi in the background may not be ideal, as it could encounter disconnections if the CPU reaches 100% during a HD video or something and grinds irssi to a halt).