Consider how many mIRC-based spam/pr0n/infected-website-advertising drones there are ALREADY on IRC
You are right about the numbers out there. While I disagree that mIRC would be a worthy packet tool on its own (it wouldnt loop fast enough), if you had several clients at your command, and many people undoubtably do, you can probably send all the packets you would want to from any 1 ip anyway without any significant slowdown in a victim's mIRC client.
In fact, I have already seen UDP flood scripts for such distributed attacks, but I have no idea if they actually work.