Family lore has it that William was a farmer's son from Co. Cork who travelled to Wexford to take part in the 1798 Rebellion and was wounded at Vinegar Hill. Apparently he managed to evade capture by making his way 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. I am told that the farmer's family was Protestant and that Mary was much older than William. Apparently Mary's brother, i.e., the farmer who took in William, emigrated to the United States and, over 100 years later, in 1910, a descendant of this farmer, who was a Catholic priest, travelled to Ireland where he met family members in Clonmel. Presumably these included William's grandson William who was living in Clonmel at that time. It is interesting that a descendant of the farmer, who was Protestant, became a priest. Perhaps the Protestant connection was with another member of the Murphy family, i.e. perhaps the farmer himself was not a Protestant.
The dates above provided by the family do not, however, tie in with the story of William's participation in the 1798 Rebellion and his flight into Carlow. 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.