This explains it a bit IQ
Basically you have the Rangers fans singing a song about the Irish potato famine and saying the irish should go home aimed at Celtic fans.
Never heard Celtic fans turn it on its head until someone showed me that clip and it amused me.
Hope that helps a bit, to explain the whole Celtic/Rangers thing would take a while, suffice to say they don't like each other !!!!!!!!!
YouTube - bbc piece on the famine song