Dublin grew around two strategic points near the confluence of the River Liffey and River Poddle, a ford across the Liffey, Áth Cliath ("Ford of the Hurdles") and a hill just right for a Celtic ring-fort1 or a castle overlooking a dark tidal pool (Dubh linn) on the Poddle, a natural harbor. Early Irish Celts would have shied away from the coast, to avoid raids by sea, but the ford carried an important road (slighe) connecting the kingdom of Meath with the seat of the Irish High King at Tara.
Raids into Ireland by Viking adventurers began in 795. Occasionally they would build camps (Irish name: longphorts) in which to spend the to winter; in 837, a fleet of 65 ships entered in the tidal pool on the Poddle, using it as a base for their raids. Soon after, they built a longphort on the hill next to it. A decade or so later, these Finn Gaill from Norway were replaced by a different band of Vikings, the Dubh Gaill from Denmark.
Large caches of silver found near Dublin indicate that the Irish wool trade was hardly what the Vikings had in mind: It is now thought that Dublin was a vast slave market. Pictish, Scots, Irish, and Anglo-Saxon captives were brought here for transshipment to Norway, Russia, Moorish Spain, and North African Islamic kingdoms. And, a chilling foretaste of what was to happen in Africa a thousand years later, feuding Irish chieftains would bring their captives to Dublin to be sold. After the 870 fall of Dumbarton Rock in Scotland, Olaf the White took 200 ships filled with captives back to Dublin.
Olaf the White ("Amilab") is the first person given the title "King of Dublin". He and Ivarr the Boneless ("Ímar") (853-873) ruled jointly for awhile, but by the time Ivarr and his brothers left to conquer England, Olaf vanished from history. Ivarr used Dublin as the base for his conquests of Northumbria, Mercia, East Anglia, and Strathclyde over the succeeding decades.
Ivarr was followed by a succession of chieftans, whose reigns were all cut short by others seeking ascendancy. Bardr, Sigfred, Sihtric, and Sigfred all lasted less than two years before they were killed or deposed. Ivarr II's reign began around 896, but the Vikings were expelled from Dublin in 902 by a combined force from Leinster and Brega. The Norse moved their operations back across the Irish Sea, taking over Jorvik, retaking Mercia from the English, and esablishing bases in northern Wales.
Sihtric "The Blind" ("Sitriuc Cáech") returned in 917 and re-established Viking control over the town. This version of Viking Dublin may have actually consisted of an Irish settlement near Ath Cliath, with the Vikings ruling from their hill. From this point, Norse control of Ireland grew until something you could actually call a "kingdom" existed, centered upon Dublin. Despite this, Viking wanderlust had not been quenched, and they were always distracting themselves with expeditions into Great Britain.
Sihtric left Dublin in 920 to take over the Kingdom of Jorvik, leaving his brother Guthfrith in charge. Guthfrith likewise left in 933, to be replaced by Olaf Sihtricson (Amlaíb Cúarán). Olaf was thrown out of Dublin on at least two occasions, the first in 936 when Donnchad Donn mac Flainn, king of Tara, burned the settlement. Olaf did not return until 945, but he, too, decided to take over the Danelaw, leaving in 946 to become King of Jorvik. He left his brother Guthfrith Sihtricson to rule in his stead. Olaf returned to Dublin in 953 after he was kicked out of York by Erik Bloodaxe. The King of Tara defeated another Norse king Olaf in 980, and power was shared between its Irish and Norse inhabitants. Coins from this time bear the mark "Diflin" of "Dyflinn".
The final decline of the Norse in Ireland is matched by the rise of Brian Boru, who expanded his domain of Dál Cais in County Clare to the overlordship of all of Munster. Brian Boru levied taxes on the Viking coastal towns, playing one off against another.
Brian and his rivals frequently sought aid from the Norse in Dublin, which allied itself first with one side, then the other. In 998, Máel Sechnaill, King of Tara, helped Brian Boru attack Dublin. In 1000, however, Máel Sechnaill rose against Brian Boru, with assistance from the Vikings. Brian Boru defeated them all, and sacked and burned Dublin the same year. Sihtric was allowed to return after providing hostages and accepting Boru's overlordship. In 1012, however, the Leinstermen rose against Boru again, allied themselves with Dublin again, and were defeated again. And now, only Dublin remained a threat to the Emperor of the Irish. He attacked the town On Christmas Day, 1013, but was held off by Sihtric and Mael Morda. The following year, Brian Boru attacked from the sea. Although Brian Boru was killed during the Battle of Clontarf, the Vikings were forced to accept Irish overlordship. By the time of Clontarf, the ford was replaced with the first bridge across the Liffey.
In 1160, Dermot MacMurrough ("Diarmait MacMurchada") was ousted from the throne of Leinster, whose capital was now Dublin. He promised lifelong fealty to English king Henry II for assistance in getting his throne back. Henry waved him off, having better things to do, such as murdering Thomas a'Becket, but Pope Adrian IV was pushing him to convert Ireland, still following the Celtic Christian church, to Catholicism. Henry allowed MacMurchada to go recruiting for his army, and Diarmait had numerous landless Norman knights to choose from. The Norman mercenaries, led by Richard Fitzgilbert de Clare (better known as "Strongbow"), did their job well. High King Rory O'Connor laid siege to Dublin but his army was scattered by a Norman breakout.
Diarmait, back on his throne, granted the knights lands in Ireland and hinted at making fitzGilbert his heir. At this, Henry became alarmed at what was going on. In 1174, he arrived in Dublin to receive the fealty of all six kings of Ireland in a wattle palace erected especially for the occasion. And so Henry became the nominal High King of Ireland, and Anglo-Norman settlement in Ireland was permanent. The remaining Norse were forced to move north of the Liffey (The northwest part of the city still has a district named Fingal). One by one, native Irish lords were replaced by Normans.
Dublin was the center of English authority on the island. In 1204 King John ordered the construction of Dublin castle, on top of the hill right next to the tide pool where the Vikings had first wintered their ships.
In 1315, King Robert I "The Bruce" of Scotland expanded his war with the English, sending his brother Edward Bruce on an expedition into Ireland. The Scottish invasion was a disaster for Ireland, coming as it did in the middle of the country's worst famine.
In 1394 Richard II traveled to Dublin to receive fealty from the Irish lords again. His prolonged stay gave one of the rival Lancastrians the opportunity to take control of England; Richard was imprisoned in the Tower of London and beheaded, and his rival crowned himself Henry IV.
The Wars of the Roses and the Hundred Years' War kept the English busy through the 15th century, and their authority shrunk to a 'Pale' around Dublin (although the independent barons of Ireland were still descendants of Strongbow's army).
In 1534, Henry VIII summoned the Earl of Kildare to London to face a charge of treason. Upon hearing rumors that the Earl had been beheaded, his son, 'Silken Thomas' Fitzgerald, attacked Dublin and laid siege to its castle, cutting Archbishop John White to bits when he tried to escape. Fitzgerald was eventually pinned up in Maynooth castle in County Kildaire, and hanged at Tyburn after he surrendered.
Henry VIII's rebellion against the Catholic Church would have far-reaching implications for the people of Ireland. The rebellion was for reasons of statecraft rather than religion, and most Irish chose to remain Catholic. Because of this, Henry and Elizabeth I embarked on extending their control over the entire island. Dublin Castle became the center, the garrison, the symbol of this occupation.
In 1592, a group of prominent Dublin citizens received a charter from Elizabeth for a college, and Trinity College was born among the ruins of the All Hallows Monastery.
The English Civil war had its echoes in Ireland, In 1641, Phelim O'Neill tried, without success, to capture Dublin. As the New Model Army gained control of England, Ireland remained a Royalist hotbed. In 1649, Charles I's viceroy, the Duke of Ormond, tried to take the city from Parliamentary forces, but was turned back at the Battle of Rathmines. Oliver Cromwell arrived later in the year to oversee the pacification of Ireland.
During the Restoration period Dublin grew from a small walled town into a true city, spurred by The Duke of Ormond, now Charles II's viceroy. A 1684 fire which destroyed Dublin Castle did not stop the city's growth, and by 1700, Dublin was the second largest city in the British Empire.
In 1689 after the Glorious Revolution, James II fled to Ireland. Hoping to use it as a base for the reconquest of Britain from William III, he housed his soldiers in the dormitories of Trinity College. The Battle of the Boyne ruined James's plans, and he fled to France.
The 18th Century saw Dublin grow into a true metropolis, being the center of British extraction of wealth from the country. 1798 saw the revolt of the United Irishmen break out in Dublin. The revolt was soon crushed, and served only to spur George III and his ministers to complete the final assertion of authority over the island, the 1802 Act of Union.
Finally, in 1921, Dublin became the capital of the new Republic of Ireland. Today, with a population of over 1,000,000, Dublin is the primate city of Ireland.
Not bad for having never been there, I think. Sources include:
Viking Network Ireland
http://www.ncte.ie/viking
Simon Schama, "Ireland and the Invasions by the English and Scots, c. 1170 - 1320".
http://www.bbc.co.uk/history/state/nations/schama_ireland01.shtml
Dublin Castle History
http://www.dublincastle.ie/history.html
C. Litton Falkiner, Sketch of the History of Dublin.
http://indigo.ie/~kfinlay/dublinsketch.html
The University of Dublin, Trinity College: A History of Trinity College
http://www.tcd.ie/General/tcd_hist.html
1We will never know whether there was actually a ring-fort or not, as history
never mentioned it, and later building would have erased all traces.
Please read aneurin's wonderful writeup The Cambro-Norman invasion of Ireland.