Irssi runs on Unix-like systems. If you download the source code, you have to compile it. If you do not want to compile it, you can try binary packages.
Current release source code download:
You should verify the signature file using
gpg --verify to make sure that the download was not tampered with.
Alternatively, the current Git development source can be found on Codeberg.
Then you type the following commands in your shell:
tar xJf irssi-*.tar.xz cd irssi-* meson Build ninja -C Build && sudo ninja -C Build install
Some more details can be found in the INSTALL file.
Many distributions have an Irssi package! Search for
irssi in your package manager of choice.
The openSUSE Build Service is creating binary builds for popular Linux distributions (Debian, Fedora). These can be installed with root permissions using the distribution package manager.