The problem :

My system is ubuntu with an UTF-8 encoding but most of the IRC channels wher I live use ISO_8859-1 or ISO_8859-15, so what can I do ?

The possible solution :

I ran irssi in a screen: like this :

screen -U -m irssi -c mon_network_ou_server_qui_va_bien

then in irssi I ran these commands :

/set recode ON
/set recode_out_default_charset ISO_8859-1
/set recode_fallback utf-8
/set recode_autodetect_utf8 ON
/recode add #channel_name ISO_8859-15

The last line has to be repeated for each channel (in the channel window)