These days I have to point new users to Quassel; there's the 'monolithic' version that behaves like old IRC clients, and there's the client/server version where the quassel "server" acts as a client and various quassel clients connect to that.

Note that it doesn't support mobile devices (in theory someone could submit a pull request to change that, but they'd probably need to write an additional QT GUI that worked correctly for them).

https://quassel-irc.org/about