The Irish surname Fitzsimons is an Anglo-Norman French patronymic from the personal name Simon. The name is also found in the Gaelicized form Mac Síomóin. The earliest settlers of the name in Ireland accompanied John De Courcy to County Down in 1177. Others of the same name followed the Prendergasts to County Mayo early in the next century. The most important line of this family came from Simonshide, given as being in Hertfordshire but probably Durham, and settled in Dublin in 1323.