The only thing I'd add to Zak's post is that you have to have normal binocular vision for any occluded eye scope to work.
In the movie Black Hawk Down, they used an older version of the Aimpoint and they were mounted to the carry handle. This gave a very high mount, but you could use the irons through the channel of the mount. I'm just guessing, because I don't know which mount they used. But the common mounts that are available allow you to use the irons.
I'd like to say that there are tradeoffs and compromises with everything. Yes, if you mount a red dot sight cowitnessed with the irons, you will have to remove it if the occular lens becomes obscured. But let me ask you this; is it faster and more efficient to wipe the mud off quickly and stay in the fight, or can you remove and discard it faster?
I'd bet that you could wipe enough mud off your occular lens to shoot faster then you could remove the sight, or even change a magazine. If you are in such a close quarters battle that even taking that much time out of the fight will cost you your life, you'll most likely continue to fire without the sight.
Ever fired a rifle with a rear peep sight in the driving rain? A lot more of a pain in the you-know-what then wiping the mud off the occular lens of your Aimpoint. Another what if to worry about!
Jeff