From Irish dubh "black" "dark" referring to a dark tidal pool where the River Poddle entered the Liffey on the site of the Castle Gardens at the rear of Dublin Castle.
From Irish Duibhlinn itself from Middle Irish Dublind meaning "black pool", derived from dub meaning "black, dark" and lind meaning "pool, lake". The name originally referred to a tidal pool where the River Poddle entered the Liffey in part of what is now Dublin Castle. This is the name of the capital city of Ireland.