irb: 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/irb - from package: ruby-irb

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

On Debian possible default full paths are:

/usr/bin/irb - from package: ruby
/usr/lib/ruby-standalone/bin/irb - from package: ruby-standalone

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

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