My family came from Co. Cavan, and emigrated to America in 1847, ending-up in Iowa in 1850 where they farmed in Fayette County. The name in an 1868 map of the township where they lived is spelled Fitzsimons, but is Fitzsimmons in an 1879 map, and has been that way since, and the family was and is a Catholic family.

I've been to an old Catholic cemetery in Hannahstown, County Antrim where there are Fitzsimmons stones. The IRA section in Milltown Cemetery (Catholic) in Belfast has a Fitzsimmons stone.

I visited an old Fitzsimons man in Mullagh, Co. Cavan, and he told me with much winking, that there are Fitzsimons and Fitzsimmons. I took it that he meant Fitzsimons is the Catholic spelling and Fitzsimmons the Protestant spelling. Fitzsimons seems nearly universal in the telephone directory for the Republic of Ireland, but when the name travels to America, it is almost impossible to keep it spelled Fitzsimons.