The best replacement for irc is ibc.

The best alternatives to irc are rated as following:
★★★★★  ibc
★★★★★  hrc
★★★  hrc cable
★★★  im
★★★  jabber
★★★  std
★★  xmpp
★★  icq
★  usenet
★  hipchat
★  campfire
★  hud
★  phrf
★  mirc
★  wrc
★  torrent
  torrents
  p2p
  hrc time warner cable
  irs

Your alternative for irc is not listed? Suggest!
Speaking about irc, one probably shouldn't forget about adium.

Other search terms related to irc and their ratings are:
★★★★★ adium
★★★★ trillian
★★★★ invision
★★ downloading
★ dcc
★ mirc
You have in mind a companion for irc and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of irc in all years. Click on graph for more.
irc
ibc
hrc