Weechat must be built with the ruby option enabled, if using homebrew:
$ brew install weechat --with-ruby
Install terminal-notifier or ruby_gntp:
$ gem install weechat terminal-notifier
or
$ gem install weechat ruby_gntp
Download weechat-notifcations from this repo:
$ mkdir -p ~/.weechat/ruby/autoload && cd ~/.weechat/ruby/autoload
$ git clone git@github.com:fabian-markaban/weechat-notifications.git weechat-notifications && ln -s weechat-notifications/notifications.rb notifications.rb