The broad, flat thoroughfares of the city centre follow the outline of what were once two marshy islands in the river Lee, separated by channels and surrounded by wetland. Those watercourses were gradually covered over during the eighteenth century, and the city's familiar grid took the shape it broadly retains now. It is a strange thing to consider: that North Main Street and South Main Street exist where they do because bridges once had to connect islands to dry land, and that the streets themselves are a kind of fossilised map of a drowned geography.

The earliest settlement here was the monastery of St Fin Barre, established in the sixth or early seventh century, almost certainly on the higher ground to the south of the river. Vikings are recorded in the area from 846 AD, and their presence is thought to have been concentrated on the southern bank of the Lee, roughly opposite where the South Gate Bridge now stands, though excavations in that area have so far produced nothing earlier than the twelfth century. The Anglo-Normans formalised the town's island structure: the southern island was enclosed within walls by 1182 and reserved to the Crown, while the northern island, then known as Dungarvan, was treated as a suburb before being brought within the walled boundary during the thirteenth century. The proliferation of religious foundations in the late twelfth and thirteenth centuries points to a period of considerable growth. That expansion contracted sharply; by the sixteenth century the city appears to have shrunk back almost entirely within its medieval walls. Recovery came after 1550, driven by overseas trade, and customs records from 1664 and 1668 show Cork functioning as Ireland's second port after Dublin. The Williamite siege of 1690 caused significant damage, including a large breach in the south-eastern section of the city wall, yet the broader seventeenth and eighteenth centuries were periods of expansion, with marshland steadily reclaimed to the east and west as the city pressed outward from its island origins.