This relative newcomer has caught up to Ircle in features, and is now the most actively developed client, with regular updates going back many years. It appeals to people who prefer a more streamlined interface. Everything for each channel is contained in one window. Easy to set up and configure, stable. Uniquely uses the scripting language from the original ircII client, so you can tap into the wealth of existing ircII scripts, and now also supports Ircle's Applescript scripts.
Guardian feature permits adult control, for what that's worth.
ShadowIRC is designed to be a lean and mean client. Compared to Ircle or Snak, ShadowIRC trades a lot of features and conveniences for speed and stability. The most unique feature is the use of pre-compiled C scripts, which theoretically run faster and allow more extensive customization than conventional scripts. ShadowIRC is newer than Ircle and Snak, and development seems to have slowed down. On the other hand, it recently became freeware and open source, both unique among Mac IRC clients.