Bunkerwaiss,
You can host an ircd on any type of connection. It's not hard to have 50 or more users even on a 56k dial-up connection. In fact, 95% of the networks that exist can be hosted on a 56k connection with very little lag, but you definately don't want to piss off any script kiddies. Most of todays "admins" don't understand exactly how little IRC actually uses.
As Jobe says it does matter what the users are doing and where on the network the users are as to how much bandwidth will be used. however, I'd like you to know that DALnet hosts between 35,000-45,000 users and each server uses about 300kbps sustained. Which will saturatate most home DSL connections with no problems, but there are no more than 7 other networks our of 4500 in the world that has more users, so any tiny speck of a userbase you have can easily be handled by a rather small amount of bandwidth.
Please understand that these figures are approximate and under normal conditions.
[Edited on 17-10-2007 by katsklaw]