I would suggest Snak , it is the best all-around client I've used. Shareware, but well worth the price. If you don't register after 30 days(?) it will limit your session lengths to 30 minutes.
Another one is Ircle and there are a few others out there.
A good software resource is Tucows, the navigation there leaves a bit to be desired, but their search function is helpful.
try searching the forums; this question pops up about once a month. my favorites are colloquy and x-chat aqua. both are free and have dcc, although each is more suitable for some situations, so try them both.
chatzilla for DCC? Aqua-X is the best `real` IRC client for OSX, since it's really a port of the best client next to mIRC (windows only). I've been using Conversation a lot lately, but it's no good for multiple channels/servers and I don't think is supports DCC at all.
x-chat aqua is so far the best i've seen (i'm also looking for a good client). i refuse to pay for shareware, so i'm not bothered with looking at snak. anyway, x-chat aqua is good for dcc and chat purposes, it could be a little better looking ... but people who use irc don't usually care.