USC fans and I will tell you why. Their fans share many qualities of Irish it is true (diehard fans, etc.), but they have no sense of history. Being from LA, I can tell you that most USC fans are just bandwagon jerks because USC is basically the closest they get to professional football since no pro team currently resides in Los Angeles. They know Reggie Bush, Matt Leinart, and Pete Carroll. And being at their stadium is always dangerous because, for one it is in a rough area, and secondly because they have no manners whatsoever. At the game this last year, I was spit on walking to the stadium. My friend, while exiting the stadium, was threatened with bodily harm for wearing the wrong colors. I saw a female Irish fan carrying her baby while pushing the baby's stroller in front of her and it hit a pebble and fell over. Not one USC fan tried to help her, instead walking over her as she struggled to hold her baby and get the stroller upright again at the same time. Some ND fan ran over and lifted it immediately.
That's my biggest complaint. Most Irish fans, because of their undying love for the team, can name you all of the Heisman winners or what years we won NC's. Heck, most USC fans don't even know that USC is a Methodist university. When SC fans visit ND, I see USC fans all over the campus taking pictures of the various amazing things there are to witness, and I see ND fans be nothing but welcoming. The respect just doesn't go both ways.