I was still confused so I googled and found this on another msg board.

"July 12th is the day the loyalists celebrate the defeat of the catholics by william of orange.

They like to do this by parading through catholic neighborhoods in a marching band formation, to rub their nose in the fact that their kin folk were killed. Obviously the catholics do not appreciate this, and say if they want to march, they should not be doing it in their neighborhood. Its akin to the KKK holding a white pride rally in Harlem, would not be greeted with open arms.

And so the petrol bombing and riots ensue"

That sounds scary, I wouldn't want to be anywhere near all that.