I will go against the current and say Irvin wins this. Irvin knows he is an underdog and has nothing to lose. He's been in the octagon enough times so he will be relaxed as well. I see Irvin going out and throwing bombs and catching Silva.
Also, fighting Silva in his first LHW bout is not the most attractive proposition. Being the main event is obviously a great thing but at the same time, winning against a MW will not give you a title shot and losing against Silva will only put you down even further away from a title shot. I think Irvin took this fight because he's been in the UFC for a long time and he knows he won't get a high profile fight like this being the gate keeper of the LHW. I guess we'll never know the real reasons why some people turned the fight down but there are plenty of reasons why someone would.