It does irritate me when people do pass me on a MUP without announcing themselves. It also really bothers me on singletrack. I can generally hear people coming so it's not really a safety issue, I just am easily offended
. Passing without notification doesn't bother me at all on the road. Honestly, people very rarely pass me though (which is not meant as a brag and is likely true of most people on this board).
I always announce myself when passing, especially on a MUP. I do this both for self-preservation reasons and for etiquette reasons.