Family lore has it that William came from Co. Wexford. Apparently, during the 1798 Rebellion, in which he lost two brothers and he himself was wounded, he escaped capture by fleeing across the Blackstairs Mountains into Co. Carlow. The story goes that he was taken in by a local farmer, also named Murphy, and nursed back to health. William subsequently married the farmer's sister Mary. According to one family source the farmer's family was Protestant and William's wife was older than him.
However, dates provided by the family do not tie in with this story. William was supposedly born about 1789 which would have made him too young to fight in the rebellion, and Mary, we are told, was born about 1790 which means she was not an older woman. Further checking, e.g., consulting church records, would need to be carried out in this regard.
Murphy is the anglicised version of two Irish surnames, 'Ó Murchadha' (in modern Irish 'Ó Murchu') and 'Mac Murchadha', both derived from the popular early Irish personal name Murchadh, meaning "sea-warrior". Mac Murchadha ("son of Murchadh") is exclusive to Ulster where they were originally based in present-day Co. Tyrone but were driven out and settled in Co. Armagh where the Ulster Murphys are now most numerous. The Murphy name is also common in counties Fermanagh and Monaghan. Elsewhere in Ireland, the Ó Murchadha (descendant of Murchadh) name arose separately in at least three distinct areas, in Cork, Roscommon and Wexford. The most prominent of these were the Wexford Uí Murchadha. These took their surname from Murchadh or Murrough, grandfather of Dermot MacMurrough, King of Leinster. Their territory lay in the barony of Ballaghkeen in Co. Wexford, their main seats being Morriscastle ("Ó Murchu's Castle"), Toberlamina, Oulart and Oularteigh. In the late sixteenth century, however, their chief, Donal Mor O'Morchoe (as the name was then anglicised) was overthrown, and all of his territory, with the exception of land in Oularteigh, was confiscated. Most of his followers were scattered and settled in the surrounding counties, in Kilkenny and Carlow particularly.
Note that in some older references to Knocklonogad, its parish is given as Sliguff rather than Garryhill. Also note that that I have seen Knocklonogad spelled as Knocklonegad and Knocklonagad.