The city of Dublin is the entire area administered by Dublin City Council. However, when most people talk about "Dublin," they also refer to the contiguous suburban areas that run into the adjacent local authority areas of Dun Laoghaire-Rathdown, Fingal, and South Dublin. This area is sometimes known as "Urban Dublin" or the "Dublin Metropolitan Area."

The population of the administrative area controlled by Dublin City Council was 505,739 at the census of 2006. At the same census the Dublin Region population was 1,186,159, and the Greater Dublin Area 1,661,185. (estimated by the CSO to reach 2.1 million by 2021). Today, approximately 40% of the population of Ireland live within a 100km (60mi) fan radius of this east coast city. A person from Dublin is known as a Dubliner or colloquially as a Dub, or, pejoratively, a Jackeen.

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.

The writings of the Greek astronomer and cartographer Ptolemy, provide perhaps the earliest reference to Dublin. In around 140 CE, he referred to a settlement he called Eblana Civitas. The settlement "Dubh Linn" dates perhaps as far back as the first century BCE and later a monastery was built there, though the town was established in about 841 by the Norse. "Baile Átha Cliath" or simply "Áth Cliath" was founded in 988, and the two towns eventually became one.

The modern city retains the Anglicised Irish name of the former and the original Irish name of the latter. After the Norman invasion of Ireland, Dublin became Ireland's capital, with much of the power centring on Dublin Castle until independence. From the 14th to late 16th centuries Dublin and the surrounding area, known as the Pale, formed the largest area of Ireland under government control.

From the 17th century, the city expanded rapidly helped by the Wide Streets Commission. Georgian Dublin was, for a time, the second city of the British Empire after London. Much of Dublin's most notable architecture dates from this time. The Easter Rising of 1916 left the capital in an unstable situation and the Anglo-Irish War and Irish Civil War left it in ruins, with many of its finest buildings destroyed. The Irish Free State rebuilt many of the buildings and moved parliament to Leinster House, but took no bold tasks such as remodelling. After The Emergency (World War II), Dublin remained a capital out of time: modernisation was slow, but finally the 1960s saw change begin. In recent years the infrastructure of Dublin has changed immensely, with enormous private and state development of housing, transport, and business. Some well-known Dublin street corners are still named for the pub or business that used to occupy the site before closure or redevelopment.

Since the beginning of English rule in the 12th century, the city has served as the capital of the island of Ireland in the varying geopolitical entities:

From 1922, following the partition of Ireland, it served as the capital of the Irish Free State (1922–1937) and now as the capital of the Republic of Ireland. (Many of these states co-existed or competed within the same timeframe as rivals within either British or Irish constitutional theory.)

Traditionally, a north-south division has existed in Dublin with the dividing line being the River Liffey. The Northside is seen by some as working-class, while the Southside is seen as middle and upper middle class. But this is not a clear divide by any means. Dublin postal districts have odd numbers for districts on the Northside — for example, Phibsboro is in Dublin 7 — and even numbers for the Southside — for example, Sandymount is in Dublin 4. An exception to the rule is Dublin 8, which straddles the river.

This division dates back centuries, certainly to the point when the Earl of Kildare built his residence on the then less-regarded Southside. When asked why he was building on the Southside, he replied "Where I go, fashion follows me," and he was promptly followed by most other Irish peers.

The Northside/Southside divide is punctuated by examples of Dublin "sub-culture" stereotypes, with upper-middle class constituents seen as tending towards an accent and demeanour synonymous with (but not exclusive to) the Dublin 4 postcode on the Southside and working-class Dubliners seen as tending towards accents and demeanour associated with (but not exclusive to) Northside and inner-city Dublin neighbourhoods.

This simplification of economic and social communities in Dublin ("Southside rich, liberal and snobby" / "Northside poor, industrial and common") does not survive more than a few real-world examples, however. For example, the President of Ireland's residence, Áras an Uachtaráin, is on the Northside, although its postal district is Dublin 8, a Southside number. Similarly, some of Dublin's working-class suburbs such as Tallaght, Dolphin's Barn, Crumlin, Inchicore, Ringsend, Irishtown, Clondalkin, and Ballyfermot, are south of the river. Wealthy suburbs such as Castleknock, Clontarf, Glasnevin, Howth, Malahide, Portmarnock, and Sutton are on the Northside. Areas of the north inner city such as Smithfield, the IFSC, and Spencer Dock are also associated with affluence.

The north-south divide has mellowed considerably in the past number of years. This is primarily due to the favourable economic conditions currently in Ireland and the emergence of the Celtic Tiger economy in Ireland. Correspondingly, Dublin has progressed to become one of the wealthiest cities in Europe.

The economic divide in Dublin is east-west as well as north-south, the east side generally being wealthier than the west. There are significant social divisions between the coastal suburbs in the east of the city, including those on the Northside, and the newer developments further to the west.