irkerhook: 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 Debian possible default full paths are:

/usr/bin/irkerhook - from package: irker

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

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