I wanted to compile FiSH for irssi on Ubuntu Gutsy, but http://fish.sekure.us was offline. Now that I checked it again, it's back. So perhaps you better follow the walkthrough on http://fish.sekure.us/forum/viewtopic.php?t=145 . For my own reference and for the case the website is down again I'm posting here what I did to get it to work. This should work for every Debian based distribution

1. Getting everything we need

fire up a terminal

sudo apt-get install irssi (if you don't have it already)

sudo apt-get install build-essentials libglib1.2 libglib1.2-dev libglib2.0-0 libglib2.0-dev

2. Now we need to download the FiSH and irssi source

wget http://fish.sekure.us/irssi/FiSH-irssi.v0.99-source.zip

wget http://irssi.org/files/irssi-0.8.12.tar.bz2

3. Unpacking the archives

unzip FiSH-irssi.v0.99-source.zip

tar xjf irssi-0.8.12.tar.bz2

4. Create the necessary directories

cd FiSH-irssi.v0.99-source

mkdir MIRACL;cd MIRACL

cp ../mir_amd64 amd64

cp ../mir_macosx macosx

5. Getting MIRACL, needed for compiling FiSH

wget http://indigo.ie/~mscott/miracl.zip

unzip -j -aa -L miracl.zip

bash linux

cp miracl.a ../

cd ..

6. Adjusting the FiSH Makefile ...

nano Makefile

set glib_dir and irssi_dir to the following parameters:

glib_dir = /usr/lib/glib/include
irssi_dir = /home/username/irssi-0.8.12

Create a symlink so that 1 library will be included

sudo ln -s /usr/include/glib-1.2/glib.h /usr/lib/glib/include/glib.h

Now we can do make

make

You will see something like this:
-e ######################## FiSH for irssi COMPILING NOTES ########################
--------------------------------------------------------------------------------
You'll need MIRACL (http://indigo.ie/~mscott/) to compile FiSH! It is suggested
to recompile miracl.a on your target machine (compile instructions can be found
-e in miracl.zip/linux.txt). Also, make sure you are using the correct mirdef.h
-e (especially in case of exotic systems). On AMD64 systems use 'mirdef_amd64.h'
-e and have a look at miracl.zip/amd64.txt, use 'mirdef_macosx.h' for MacOSX. You
-e can test MIRACL with pk-demo. BSD fails in pk-demo.cpp and wants a '(time_t *)'
in line 54.
-e Using irssi source from: /home/username/irssi-0.8.12
-e Using glib source from: /usr/lib/glib-2.0/include (required to compile irssi)
-- Please make sure you have both installed!
e -- If you plan to use a custom password for your blow.ini (via /setinipw),
-e -- you can skip the following note!
You are adviced to change default_iniKey in FiSH.h - But remember, changing the
default blow.ini password will make your old keys in blow.ini unusable!
-e >> Usual procedure to compile FiSH:
unzip FiSH-irssi.v0.99-source.zip;cd FiSH-irssi.v0.99-source
mkdir MIRACL;cd MIRACL;cp ../mir_amd64 amd64;cp ../mir_macosx macosx
wget http://indigo.ie/~mscott/miracl.zip
-e unzip -j -aa -L miracl.zip (see linux.txt/amd64.txt for more info)
-e bash linux (or 'bash amd64' / 'bash macosx')
cp miracl.a ../;cd ..
adjust glib_dir and irssi_dir in 'Makefile'
-e make (or 'make macosx' / 'make amd64')
cp libfish.so ~/irssi/lib/irssi/modules
-------------------------------------------------------------------------------- Press ENTER to continue or CTRL+C to abort...

enter

If all goes wel, you'll get a lot of messages ending with the following:

make[1]: Leaving directory `/home/username/FiSH-irssi.v0.99-source'
rwxr-xr-x 1 username username 123243 2008-02-03 18:06 libfish.so
e Compiling finished /home/username/FiSH-irssi.v0.99-source/libfish.so

7. finishing the installation

sudo cp libfish.so /usr/lib/irssi/modules

sudo rm /usr/lib/glib/include/glib.h (remove the symlink)

8. Loading FiSH in irssi

irssi

/load fish

you'll see something like this:

-!- FiSH: Using default password to decrypt blow.ini... Try /setinipw to set a custom password.
-!- Irssi: FiSH v0.99 - encryption module for irssi loaded! URL: http://fish.sekure.us
-!- Irssi: Try /helpfish or /fishhelp for a short command overview
-!- Irssi: Loaded module fish/core

8. Getting the key

Ask someone from the channel to /keyx the key to you. When It appears in a pm, go to the status window and do

/setkey #channelname 'key'

That's all folks!