To build, you need either:

* (if using 'master' branch)... the original irssi-0.8.15 sources
  unpacked

* (if using 'build-against-fixed-rpm' branch)... rebuilt irssi and
  irssi-devel RPMs from the SRPM found here:
  http://pfrields.fedorapeople.org/packages/SRPMS/irssi-0.8.15-7.1.fc16.src.rpm

You'll also need the normal development tools such as GNU autotools
and libtool.  Then simply execute the following in a shell:

libtoolize -fci
aclocal
autoheader
automake --gnu -a -c
autoconf
# here, <lib> should be lib for i686 or lib64 for x86_64... and use
# the right path, /usr/include/irssi if you took the second option above
./configure --with-irssi=/path/to/irssi/ --prefix=/usr --libdir=/usr/<lib>
make
sudo make install

Now you should be able to do the following in irssi:

/load python
/py exec import irssi
/py exec dir(irssi)

Have fun!