irssi: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/irssi - from package: irssi

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/irssi - from package: irssi

You may install the required package with command apt-get install {package}

Error in other languages:
irssi: bevel nie gevind nie
irssi: comandă negăsită
irssi: comando não encontrado
irssi: commande introuvable
irssi: command not found
irssi: käsku ei ole
irssi: không tìm thấy lệnh
irssi: komanda nerasta
irssi: Kommando nicht gefunden.
irssi: kommandot finns inte
irssi: komut yok
irssi: nie znaleziono polecenia
irssi: níor aimsíodh an t-ordú
irssi: no se encontró la orden
irssi: no s'ha trobat l'ordre
irssi: opdracht niet gevonden
irssi: parancs nem található
irssi: perintah tidak ditemukan
irssi: príkaz nenájdený
irssi: příkaz nenalezen
irssi: команда не найдена
irssi: командата не е открита
irssi: 命令找不到
irssi: コマンドが見つかりません