Use programs such as NetLimiter to limit your speeds. There are also a few scripts out there that will allow this, I think. Of course, I've never noticed lag caused by mIRC downloading anything, so it sounds like some other problem. Even when downloading off the web, your downloads will go at full speed (and usually faster than on DCC) and won't lag anything. So, it really does sound like you have some other problem.
Of course, you shouldn't probably be downloading on mIRC. Especially nothing that is large enough to make lag a problem.