William Marshal was largely responsible for the shape of Kilkenny as we know it today. From his arrival in 1207 to his leaving in 1213, William Marshal set about transforming Kilkenny into the seat of the Lordship of Leinster.
His foundation of the port of New Ross was key to the success of Kilkenny as being situated at the meeting point of the Barrow and the Nore, the port facilitated the access of imported commodities directly to Kilkenny via the navigable Nore. 
In 1207 he issued a charter confirming the burgesses of Kilkenny in their freedoms and valuable privileges which protected and empowered them, by fixing their rent, confirming tax exemptions, and allowing them to institute and control their own weekly court, the Hundred Court, thus facilitating the economic growth of Kilkenny.
William Marshal replaced the wooden castle with a stone quadrangle castle, and to extend the size of the town he acquired the land between James’ Street and the Breaghagh River from the Bishop, Hugh de Rous, thereby making High Street his thoroughfare from the Castle to St. Canice’s Cathedral. He endowed St. John’s Abbey, 
known as ‘The Lantern of Ireland’ due to the numerous tall Gothic windows glowing with candlelight, and his sons, William and Richard, endowed the Black Abbey and St. Francis’ Abbey respectively. Outside in the hinterland, William Marshal endowed the Abbey at Graiguenamanagh and the Priory at Kells.
William Marshal’s patronage facilitated the development of Kilkenny into a thriving and prosperous town of wealthy merchants, with tenants, artisans, and servants making up a population in the mid-13th century of 2,500 to 4,000 people. The foundations of the city we know today were laid by William Marshal, The Greatest Knight Who ever Lived.
References:
Images show: William Marshal The Flower of Chivalry from the Ross Tapestry.
http://www.rostapestry.com/panel_7_william_marshall.htm
The Thriving Port of New Ross http://www.rostapestry.com/panel_13_thriving_port.htm
St. John’s Abbey http://www.historickilkenny.com/#!st.-john%275-abbey/zoom/c1pwt/dataltem-ihscqn0x2
William Marshal’s Effigy http://www.gethistory.co.uk/historical-period/medieval/normans-beyond/william-marshal-the-story-of-a-remarkable-man
Asbridge, Thomas, The Greatest Knight (London: Simon & SSchuster UK Ltd., 2015)
Bradley, John, ‘Kilkenny’, in Irish Historic Towns Atlas vol II. (Bray: Royal Irish Academy, 2005)
Crouch, David ‘Marshal, William (I)’ Oxford Dictionary of National Biography, Ed H.C.G. Matthew and Brian Harrison (Oxford: Oxford University Press, 2004) p. 822
Phelan, Margaret, M. ‘William Earl Marshal (1144 – 1219)’, in Old Kilkenny Review, N.S. Vol II No. 5 (1983)
