Compiling Irssi from SVN (subversion) repositories on Debian and Ubuntu is fairly simple. I will try to explain how in tutorial.
Note, this is probably not the most efficient way to built it from source if you are looking for a minimal system. Although if you are building other programs on your machine, you will probably find the added bulk of packages as useful in the future. It's also a far simpler way for users who are uncomfortable going through and installing individual packages.
1) Start by going to your /etc/apt/sources.lst with your preferred text editor. For example, Nano:
sudo nano /etc/apt/sources.lst
Make sure that any listing starting with is uncommented (no '#' before it), save and exit, then type:
sudo apt-get update
2) Bring in the build dependencies for irssi and subversion:
sudo apt-get build-dep irssi subversion
3) Install subversion and links or lynx for converting help files:
sudo apt-get install subversion links
4) Grab Irssi svn repos:
svn co http://svn.irssi.org/repos/irssi/trunk/ irssi
./autogen.sh # Generates configure script - Add ./configure options here
make -jN # N = Number of processors you have. For most people, adding -j3 should work fine.
sudo make install # sudo is only needed if you are installing into a directory that only root has access to, which is default
And now you're ready to go! For added functionality, consider installing screen so that you can detach and reattach your irssi session.
sudo apt-get install screen
Enjoy. Any queries/comments, please post below. Criticisms welcomed.