William Marshal’s achievements in developing the Lordship of Leinster, establishing Kilkenny as its seat, and securing the future of his dynasty were not won without being fraught with difficulties. The most gruelling of these was Meilyr FitzHenry.
FitzHenry had arrived in Ireland with the first Norman conquerors, and as his name suggests, was a grandson of Henry I through one of the King’s illegitimate sons. He was, therefore, a cousin of King John who appointed him Justiciar of Ireland in 1199. FitzHenry was grasping, avaricious and deeply unpopular with most of the Norman lords, primarily De Lacy.
It is clear that Meilyr FitzHenry regarded William Marshal as a threat to his power base. 
He was determined to get rid of him, and with King John, ‘plotted his downfall’, (Asbridge). John recalled Marshal in 1207, ostensibly to resolve matters between him and Meilyr FitzHenry.
With Marshal safely out of the country, FitzHenry ordered the burning of New Ross and the attack on Kilkenny and other of the Marshal estates. Using his diplomatic skills learned from long years at court, William managed to survive FitzHenry’s assault, and Marshal’s wife, Isabel de Clare, successfully defended Kilkenny, captured FitzHenry, took his hostages, and began the rebuilding of New Ross. 
Meilyr FitzHenry was relieved of the Justiciary, forced to give up Dunamase Castle and live the rest of his life in relative obscurity. 
William Marshal was more than a warrior and champion of the tourney circuit. He was an experienced diplomat, a statesman, The Greatest Knight Who Ever Lived.
Rererences:
Images: King John, http://alchetron.com/John,-King-of-England-1051661-W
William Marshal, http://www.threeriver.org/marshal/marshal_1.html
The Building of St. Mary’s, New Ross, http://rostapestrylovers.com/gallery/item/2-gothic-glory-the-building-of-the-parish-church-of-st-mary-s-in-1210
Dunamase Castle, http://picturingireland.bogspot.ie/2015/10/magical-places-rock-of-dunamase-co-laois.html
Secondary Sources:
Asbridge, Thomas, The Greatest Knight (London: Simon & Schuster Uk Ltd., 2015) p. 301.
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.
