As you may know, Quassel is an open-source, cross-platform IRC client developed in Qt. It is similar to the WeeChat IRC client, but it also has a GUI.
The latest version available is Quassel 0.12.3, which has been released a while ago, coming with changes.
In order to successfully install Quassel on Ubuntu Ubuntu 16.04 Xenial Xerus, Ubuntu 15.10 Wily Werewolf, Ubuntu 14.04 Trusty Tahr, Linux Mint 17.x, Elementary OS 0.3 Freya and other Ubuntu derivative systems:
$ sudo add-apt-repository ppa:mamarley/quassel
$ sudo apt-get update
$ sudo apt-get install quassel
Optional, to remove quassel, do:
$ sudo apt-get remove quassel