The history of Limerick stretches back to its establishment by Vikings as a walled city on King's Island (an island in the River Shannon) in 812, and to the granting of Limerick's city charter in 1197.

King John ordered the building (1200) of a great castle. The city was besieged three times in the 17th century, culminating in the famous 1691 Treaty of Limerick and the flight of the defeated Catholic leaders abroad. Much of the city was built during the following Georgian prosperity, which ended abruptly with the Act of Union in 1800. Today the city has a growing multicultural population.

Luimneach originally referred to the general area along the banks of the Shannon Estuary, which was known as Loch Luimnigh. The original pre-Viking and Viking era settlement on Kings Island was known in the annals as Inis Sibhtonn and Inis an Ghaill Duibh^{[citation needed]}

The name dates from at least 561, but its original meaning is unclear. Early anglicised spellings of the name are Limnigh, Limnagh, Lumnigh and Lumnagh, which are closer to the Irish spelling. There are numerous places of the same name throughout Ireland (anglicised as Luimnagh, Lumnagh, Limnagh etc.). According to P W Joyce in The Origin and History of Irish Names of Places II, the name "signifies a bare or barren spot of land". Similarly, others have suggested that the name derives from loimeanach meaning "a bare marsh" or "a spot made bare by feeding horses". The Dindsenchas (The Metrical Dindsenchas III page 274), attempted to explain the name in a number of ways, connecting it particularly with luimnigthe ("cloaked") and luimnechda ("shielded").

William Camden wrote that the Irish had been jealous about their antiquity since the deluge and were ambitious to memorialise important events for posterity. The earliest provable settlement dates from 812; however, history suggests the presence of earlier settlements in the area surrounding King's Island, the island at the historical city centre. Antiquity's map-maker, Ptolemy, produced in 150 AD the earliest map of Ireland, showing a place called "Regia" at the same site as King's Island. History also records an important battle involving Cormac mac Airt in 221 and a visit by St. Patrick in 434 to baptise an Eóganachta king, Carthann the Fair. Saint Munchin, the first bishop of Limerick died in 652, indicating the city was a place of some note. In 812 Danes sailed up the Shannon and pillaged the town, burned the monastery of Mungret but were forced to flee when the Irish attacked and killed many of their number.

The earliest record of Vikings at Limerick is in 845, reported by the Annals of Ulster, and there are intermittent reports of Vikings in the region later in the 9th century. Permanent settlement on the site of modern Limerick had begun by 922. In that year a Viking jarl or prince called Tomrair mac Ailchi—Thórir Helgason—led the Limerick fleet on raids along the River Shannon, from the lake of Lough Derg to the lake of Lough Ree, pillaging ecclesiastical settlements. Two years later, the Dublin Vikings led by Gofraid ua Ímair attacked Limerick, but were driven off. The war between Dublin and Limerick continued until 937 when the Dubliners, now led by Gofraid's son Amlaíb, captured Limerick's king Amlaíb Cenncairech and for some reason destroyed his fleet. However, no battle is actually recorded and so a traditional interpretation has been that Amlaíb mac Gofraid was actually recruiting Amlaíb of Limerick for his upcoming conflict with Athelstan of England, which would turn out be the famous Battle of Brunanburh. The 920s and 930s are regarded as the height of Norse power in Ireland and only Limerick rivalled Dublin during this time.

The last Norse King of Limerick was Ivar of Limerick, who features prominently as an enemy of Mathgamain mac Cennétig and later his famous brother Brian Boru in the Cogad Gáedel re Gallaib. He and his allies were defeated by the Dál gCais, and after slaying Ivar, Brian would annex Norse Limerick and begin to make it the new capital of his kingdom. The power of the Norsemen never recovered, and they were reduced to the level of a minor clan; however, they often played pivotal parts in the endless power struggles of the next few centuries.

Brian Boru's son, Donnchad (Donough), was routed by Diarmait mac Maíl na mBó, King of Leinster, in the year 1058 when Limerick was burned, a punishment he repeated five years later. A year later Diarmait defeated Donnchad again forcing him to flee overseas and installing Turlough instead. Obviously Limerick was of great importance as evidenced by being a contentious issue between neighbouring chieftains and foreigners who burned and pillaged the city. Brian Boru's sons were usually called Kings of north Munster though their reigns were rather disturbed until 1164 when Donnchad mac Briain became King of Munster. His reign was successful, founding monasteries and nunneries, constructing several monuments, including a church on the Rock of Cashel, and in his grant bestowing his Limerick Gothic palace to the church he styled himself King of Limerick. However the Danes were still a powerful force who were able to obtain four sequential Danish bishops concentrated by the Archbishop of Canterbury not subservient to the See of Cashel.