The CVS mirror can be setup in few very simple steps:

o Make sure you have cvs and rsync binaries available.
o Add user 'cvs' who will own all the CVS related files,
  this users home directory will keep the mirror. We assume
  /home/cvs for the sake of this document.
o Add an entry for pserver in your /etc/services similar to this:

pserver      2401/tcp                # Client/Server CVS operations

o Set an environment variable CVSROOT to point to /home/cvs, then
  execute:

cvs init

  This one will initialize your repository.

o Get the repository for the first time:

/usr/local/bin/rsync -avz --delete main.irssi.org::irssicvs /home/cvs/irssi

  Check if everything is all right.

o Setup a cronjob that'll rsync the cvs repository from main.irssi.org
  Here's an example:

15 * * * * /usr/local/bin/rsync -avz --delete main.irssi.org::irssicvs /home/cvs/irssi

o Add the following line to /etc/inetd.conf or whatever you're using:

pserver stream tcp nowait root /usr/bin/cvs cvs -f --allow-root=/home/cvs pserver

o Edit the /home/cvs/CVSROOT/passwd file, enter the following line there:

---cut here---
anonymous::cvs
---cut here---

o Create empty /home/cvs/CVSROOT/writers

touch /home/cvs/CVSROOT/writers

o Now, some testing, use the following lines:

export CVSROOT=:pserver:anonymous@your.cvs.mirror.here:/home/cvs
cvs login
(enter empty password)
cvs -z3 co irssi

If that works, then you've just got yourself a cvs mirror. In case
of any trouble check http://www.cvshome.org/ which is a great
repository of knowledge on CVS.