County Dublin, or more correctly today the Dublin Region (Réigiúin Átha Cliath), is the area that contains the city of Dublin, the capital and largest city of the Republic of Ireland as well as the largest city on the island of Ireland; and the modern counties of Dun Laoghaire-Rathdown, Fingal and South Dublin. The Dublin Regional Authority is the regional authority for the region. Dublin was one of the oldest of the Counties of Ireland to be formally structured.

The name Dublin is a derivative of "Dubh Linn" (Irish, meaning "black pool"). Historically, in the traditional Gaelic script used for the Irish language, 'bh' was written with a dot over the 'b', viz "Duḃ Linn" or "Duḃlinn." The Normans omitted the dot and spelled the name variously as "Develyn" or "Dublin." Some sources doubt this and suggest that "Dublin" is of Scandinavian origin (djúp lind or "deep pond"). However, the name "Dubh Linn" pre-dates the arrival of the Vikings in Ireland. The Old Norse (and modern Icelandic) name for Dublin is simply the words "Dubh Linn" re-spelled as if they were Old Norse: "Dyflinn" (correctly pronounced "Duev-linn").

The common name for the city in Modern Irish is "Baile Átha Cliath" (The Settlement of the Ford of the Reed Hurdles), which refers to the settlement founded in 988 by High King Mael Sechnaill II that adjoined the town of Dubh Linn proper at the Black Pool.

Dublin is located on the east coast of Ireland in the province of Leinster. The area held county status until its dissolution as such in 1994, where Section 9 Part 1(a) of the Local Government (Dublin) Act, 1993 stated that as of 1 January 1994 "the county shall cease to exist." At that time, and in response to a European Council report highlighting Ireland as the most centralised country in the European Union, it was decided that a single County Dublin was unmanageable and undemocratic from a Local Government perspective. The county was formally abolished and replaced with Dun Laoghaire-Rathdown, Fingal, and South Dublin. The Government was fully aware of the magnitude of the legislation that was being passed, most poignantly noted by TD Avril Doyle in the Dáil Éireann assembly of the 3rd of November 1993, where she declared "The Bill before us today effectively abolishes County Dublin. I am not sure whether Dubliners realise that that is what we are about today, but in effect that is the case."

At present, many organisations, state agencies, and sporting teams continue to operate on a "County Dublin" basis. Subsequently, much confusion exists around the legitimacy of the new counties - though they do have administrative county status, the only protected recognition of a county within the Republic of Ireland. County Dublin is now defined in legislation solely as the "Dublin Region" under the Local Government Act, 1991 (Regional Authorities) (Establishment) Order, 1993, and this is the terminology officially used by the four Dublin administrative councils in press releases concerning the former county area. Separate use of the term Greater Dublin Area, which consists of all of the Dublin Region and the counties of Kildare, Meath, and Wicklow has created additional confusion.