Count me as someone who has always thought of it as EAR-rop. As someone mentioned, that's based on the fact that it's short for IRRegular OPerations, so I just assume that the pronunciation from the words would carry over.
And I agree with the poster who said an IRROP would be anything that interrupts any flight (except for "routine" minor delays). I guess the hallmark for me is the need to take different flights from those scheduled, and/or getting to my destination more than a couple hours after my scheduled arrival.