mIRC is the most popular IRC Client for Windows.
License: Proprietary/Nagware
Website: https://www.mirc.com
HexChat is an IRC Client for Windows, Linux/Unix and macOS.
License: GPL
Website: https://hexchat.github.io
Irssi is a text mode IRC Client for Windows, Linux/Unix and macOS.
License: GPL
Website: https://irssi.org