Wikipedia
Fitzsimons (also spelled FitzSimons, Fitzsimmons or FitzSimmons) is an Irish surname of Norman origin. The Gaelicisation of this surname is Mac Shíomóin.
The name "FitzSymons" and its pre-standardization variants (Fitzsimons, Fitzsimmons, Fitz-Simons, etc.) is not a sept, or clan, name, but rather an individual patronymic passed down through various, yet discrete, colonial families arriving at different times in Irish history. Some families "went native" during the Gaelic revival of the 14th and 15th centuries, and many refused to endorse the Protestant Reformation. Others became important members of the Protestant Ascendancy and its supporting mittelstand. Two distinct families can be identified: those who arrived when the surname was first recorded in Ireland in 1177, attached to an adventurer seeking swordlands in Ulster, known as Sir John de Courcy of Carrickfergus Castle, earl of Ulster. These Fitzsimons are now native to the east-central seaboard of Ulster, in Lecale, Ards and Down.
In 1323, a junior member of the Fitzsymons' of Simonshide, Herefordshire, settled in Dublin. This family is thought to be distinct from the Ulster Fitzsimons. Settling in Dublin, and the north and south reaches of Dublin County, they expanded into Meath, Westmeath, King's and Queen's County of the central English Pale. This branch may have been the root of the Wexford Fitzsimons family which produced a Signer of the Declaration of Independence (or the Wexford family may have sprung from a Norman adverturer arriving with Strongbow). Of the Pale Fitzsimons, it is thought discrete branches settled at Tullynally, County Meath, the line of Sir William Johnson and 'went' native by intermarrying with the O'Reillys and MacMahons of south central Ulster. These are generally the families now with ties to County Cavan and County Longford.
The English family which sent its youngest son to Dublin in 1323 died out in the name, only the Irish branch now survives. The name "Fitzsimons" is quite common in England itself. These originating in Norfolk, Lincolnshire and Nottinghamshire are thought to be Scandinavian and of the genere Danus, as the area was settled by Danish Vikings and predate the Norman invasion in 1066 AD.