Search the core files for "ISR" and "SIGNAL". "wiring.c" is a good place to start. That's where the core comes to life.
Search the internet for...
http://www.google.com/search?q=gcc+avr+interrupt
You'll probably need some inline assembly...
http://www.google.com/search?q=gcc+avr+inline+assembly
More reference...
http://www.avrfreaks.net/
When all else fails...