The celtic road network that existed in Ireland prior to the arrival of the vikings had its nexus at the Ford of Hurdles over the Liffey. These celtic routes formed the basis of the present main routes through the city. There were two important foci in the development of the city - the Ford of the Hurdles and the intersection of the river Poddle with the Liffey - the Black Pool which gave the name Dubh linn while a settlement at the ford was known as Áth Cliath - now accepted as the Irish for Dublin.

The viking's arrived in 837 but it was not until 841 that they settled here. Once settled they carved out a small kingdom around their trading post and the area has names redolent of the viking origins - Leixlip, Howth, and Skerries. Dublin was known as Dvfflinskiri and became an important settlement in the viking world. After the defeat of the vikings by the irish at Clontarf in 1014, their power waned and the city developed along more european lines. By the 12th century, Dublin had acquired all the trappings of a medieval city - walls, cathedrals, abbeys and civic buildings.

In 1170, the city was seized by the Normans and joined the mainstream of european development. It became the centre of the norman colony in Ireland and a royal charter was granted in 1172. In 1174 , the city was granted to the men of Bristol by the King for services rendered. Guilds, a mayor and bailiffs were introduced to control the city and the Vikings were expelled to a settlement at Oxmanstown on the north side of the river - thus forming a city with two ethnic communities - a situation which existed well into the eighteenth century.

The city continued to expand dramatically through the 12th and 13th centuries when all of the important institutions were founded. The city's development was continuous with no major radical surgery performed on the street plans. The Vikings built their settlements around existing Celtic routes and positioned the future suburbs while the Normans confirmed this and extended the city. The city as developed by the Vikings was based around the ridge of high ground that runs alongside the south side of the Liffey. At this time the Liffey was a wide tidal estuary before the quays were built to confine it.