The oldest evidence of human settlement in Dublin is a set of wooden fish traps found near Spencer Dock in 2007, radiocarbon-dated to around 5,700 BC - the late Mesolithic, when small groups of hunter-gatherers worked the tidal flats at the mouth of the Liffey, setting weirs to catch fish on the incoming tide and retrieve them as the water fell. Six and a half thousand years later the Vikings arrived. In 841 they returned to the tidal pool where the small River Poddle entered the Liffey, the dark pool the Irish called Dubh Linn, and built a longphort - a fortified naval encampment that became the largest slave market in Western Europe under Norse-Gaelic rule. Brian Boru beat the Dublin Norse at Clontarf in 1014 and reduced the kingdom to vassal status. In 1170 Diarmait Mac Murchada and his Norman allies stormed the city walls during a parley, and within two years the entire kingdom had been carved into the new county of Dublin by Henry II's administration. The shire was the model on which all subsequent Irish counties would be founded.

By the early sixteenth century, English authority in Ireland had retreated from most of the island. What survived was a small area around Dublin known as the Pale - effectively the modern counties of Dublin, Meath, Louth, and the eastern part of Kildare - within which English law and custom were enforced and beyond which Gaelic chieftains ran their own affairs. Henry VIII proclaimed the Kingdom of Ireland in 1542 and the Tudor conquest gradually pushed the boundary outward; by 1603 the entire island was under English control. Eighteenth-century Dublin became one of the great Georgian cities of Europe, much of its present architectural character laid out in just fifty years between 1750 and 1800. After the 1800 Acts of Union abolished the Irish Parliament, the city went into a long decline. By 1900 Dublin had some of the worst housing conditions in Europe; in 1911 over 20,000 families in central Dublin were living in one-room tenements; on Henrietta Street, 109 people lived in a single Georgian townhouse that had been built for a viscount. The Irish Times in 1901 reported that Dublin's death rate compared 'favourably' with that of Calcutta during a bubonic plague outbreak.

On 1 January 1994 the old Dublin County Council was abolished and three new administrative counties came into being: Dun Laoghaire-Rathdown to the southeast, with the wealthiest housing in the country along its coast; South Dublin to the southwest, with the sprawling 1970s-built suburbs of Tallaght and Clondalkin; and Fingal to the north, by far the largest of the three, covering nearly half the county's land area and stretching out into the rural farmland of Skerries, Balbriggan, and Rush. Dublin City Council, with jurisdiction over the central 117.8 square kilometres of the urban core, makes a fourth authority. The TD Avril Doyle remarked at the time that 'as one born and bred in these parts of Ireland I find it rather strange that we in this House are abolishing County Dublin.' The phrase has not in fact left common usage. People still write County Dublin as their address; sporting teams, the GAA, the Garda Dublin Metropolitan Division, and Dublin Fire Brigade all still treat the county as a single unit. The legal abolition has not undone the social reality.

Approximately a quarter of County Dublin's population - around 25 percent - was born outside the Republic of Ireland. This is the highest proportion of any county in the country. Historically the largest foreign-born group was British, dominated by people from the United Kingdom who came for work, marriage, or family connection. After Poland joined the European Union in 2004, Polish migration to Ireland became one of the largest sudden migration flows in Irish history; just 188 Poles applied for Irish work permits in 1999, but by 2006 the number had reached 93,787. By the 2022 census the Indian-born population had become the fastest-growing major immigrant group, more than tripling in the previous decade to nearly 30,000 people - driven in part by the tech multinationals headquartered in Dublin's Silicon Docks and Sandyford. Ireland's first dedicated Hindu temple opened in Walkinstown in 2020. The Brazilian-born population also more than tripled between 2011 and 2022, reaching 16,441 - a function of the Brazilian government's Ciencia sem Fronteiras scholarship programme. As of October 2023, around 14,000 Ukrainian refugees under the Temporary Protection Directive were living in emergency accommodation within the county.

Dublin is Europe's largest financial centre outside Frankfurt, London, and Luxembourg. The International Financial Services Centre at the Dublin Docklands, established as a special economic zone in 1987, administers over 1.8 trillion euro of funds as of 2020 - a figure larger than the GDP of most countries on Earth. The Dublin Region's GDP in 2020 was 157.2 billion euro, larger than that of roughly 140 sovereign states; its GDP per capita of 107,808 euro is one of the highest in the European Union. Forbes has consistently ranked it among the world's most business-friendly cities. And yet, the southern boundary of the same county is dominated by the Wicklow Mountains, where Kippure (757 metres above sea level) forms the highest point. The Dublin Mountains Way, a 43-kilometre walking route from Shankill to Tallaght, crosses the upland fringe. North of the city, Fingal's flat fertile plains produce 55 percent of Ireland's fresh fruit and vegetables. Lambay Island, 5 kilometres offshore, supports one of the largest seabird colonies in Ireland and a herd of feral red-necked wallabies introduced by an eccentric Edwardian baron. The world's first commuter railway line, the Dublin and Kingstown Railway, opened from Westland Row to Dun Laoghaire in 1834 and is still in service. The county that holds Ireland's largest city also holds some of its most surprising countryside.