There is archaeological debate regarding precisely where Dublin was established by Celtic-speaking people in the 7th century. Later expanded as a Viking settlement, the Kingdom of Dublin became Ireland's principal city following the Norman invasion. The city expanded rapidly from the 17th century and was briefly the second largest city in the British Empire before the Acts of Union in 1800. Following the partition of Ireland in 1922, Dublin became the capital of the Irish Free State, later renamed Ireland.
As of 2010, Dublin was listed by the Globalization and World Cities Research Network (GaWC) as a global city, with a ranking of "Alpha-", which places it amongst the top thirty cities in the world. It is a historical and contemporary centre for education, the arts, administration, economy and industry.
The name Dublin comes from the Irish word Dubhlinn, early Classical Irish Dubhlind/Duibhlind, from dubh (/d̪uβ/, /d̪uw/, /d̪uː/) meaning "black, dark", and lind (/lʲiɲ(d̪ʲ)/) "pool", referring to a dark tidal pool. This tidal pool was located where the River Poddle entered the Liffey, on the site of the castle gardens at the rear of Dublin Castle. In Modern Irish the name is Duibhlinn, and Irish rhymes from Dublin County show that in Dublin Leinster Irish it was pronounced Duílinn (/ˈd̪ˠiːlʲiɲ/). The original pronunciation is preserved in the names for the city in other languages such as Old English Difelin, Old Norse Dyflin, modern Icelandic Dyflinn and modern Manx Divlyn as well as Welsh Dulyn. Other localities in Ireland also bear the name Duibhlinn, variously anglicized as Devlin, Divlin and Difflin. Historically, scribes using the Gaelic script wrote bh with a dot over the b, rendering Duḃlinn or Duiḃlinn. Those without knowledge of Irish omitted the dot, spelling the name as Dublin. Variations on the name are also found in traditionally Gaelic-speaking areas of Scotland (Gàidhealtachd, cognate with Irish Gaeltachta), such as An Linne Dhubh ("the black pool"), which is part of Loch Linnhe.
It is now thought that the Viking settlement was preceded by a Christian ecclesiastical settlement known as Duibhlinn, from which Dyflin took its name. Beginning in the 9th and 10th century, there were two settlements where the modern city stands. The Viking settlement of about 841, Dyflin, and a Gaelic settlement, Áth Cliath ("ford of hurdles") further up river, at the present day Father Mathew Bridge (also known as Dublin Bridge), at the bottom of Church Street. Baile Átha Cliath, meaning "town of the hurdled ford", is the common name for the city in modern Irish. Áth Cliath is a place name referring to a fording point of the River Liffey near Father Mathew Bridge. Baile Átha Cliath was an early Christian monastery, believed to have been in the area of Aungier Street, currently occupied by Whitefriar Street Carmelite Church. There are other towns of the same name, such as Àth Cliath in East Ayrshire, Scotland, which is Anglicised as Hurlford.
The area of Dublin Bay has been inhabited by humans since prehistoric times, but the writings of Ptolemy (the Greco-Roman astronomer and cartographer) in about AD 140 provide possibly the earliest reference to a settlement there. He called the settlement Eblana polis (Greek: Ἔβλανα πόλις).