Limerick derives from Luimneach, meaning "bare or barren land", a term that originally described a stretch of the Shannon estuary rather than any particular settlement. It is an oddly unflattering origin for a city with such a layered past, and it points to a recurring feature of the place: what looks unassuming on the surface conceals a great deal of turbulent history underneath. The Curragour Falls on the Shannon, for instance, would have given the site genuine strategic importance long before anyone thought to build a town here, yet only two stone axeheads and two bronze dirks can be firmly associated with the area from prehistoric times, which the Urban Survey of 1989 noted as surprisingly sparse.

The city as we know it was founded in 922 by the Norse king Tamar mac Ailche, identified tentatively as Thormodr Helgason and described in the annals as "king of an immense fleet". He landed on Inis Sibtond, the island now known as King's Island, and established a longphort there, a term for a fortified Viking harbour settlement. The location was shrewdly chosen: the island was defensible, the Shannon estuary opened onto Atlantic trade routes, and the shallows at Curragour Falls restricted rival river traffic. Within a year, his fleet had devastated Terryglass, Lorrha, Clonfert, and Clonmacnoise, and the raid on Inis Cealtra on Lough Derg was remembered two centuries later for having "drowned its shrines, relics and its books". The settlement grew into an aggressive, independent power. Leaders such as Colla mac Bairid and later Olafr Cenncairech, whose nickname translates as "scabby-head", ranged across Connacht, Meath, and beyond, and one scholar has speculated that the Hare Island hoard in Co. Westmeath, the largest gold find from Viking-age Europe, may represent wealth accumulated by the Limerick armies. That independence ended in August 937 when Olafr Gothfrithson, king of Dublin, defeated the Limerick Vikings at Lough Ree, smashed their ships, and brought Olafr Cenncairech back to Dublin in chains. A period of Dublin domination followed, then a gradual integration into Irish political life, before Mathgamain mac Cennetig seized the town in 967 and burned it to the ground. The account written two centuries later in the Cogadh Gaedhel re Gallaibh describes the plunder in vivid detail: silks, satins, gold, foreign saddles, and enslaved captives sorted on the hills of Saingel.

King's Island remains the oldest part of the city and the logical place to begin any exploration of its Norse and medieval layers. The ground level has risen considerably over the centuries, and much of what the Vikings built lies well below the present streets. The area around St Mary's Cathedral, which occupies ground with its own long history of use, rewards slow walking and close attention to the topography, particularly the way the island sits within its channels of the Shannon. The Curragour Falls, still visible upstream, are worth seeking out as the physical feature that made this particular bend in the river matter in the first place.