He was raised in County Limerick as a Roman Catholic, and at the age of sixteen he went to Paris to study medicine.
[1] He was called to the Irish bar in 1731, and in November of the same year he converted to the established Church of Ireland.
Fitzgibbon amassed a considerable personal fortune practising as a barrister and consulting lawyer, with which he purchased Mountshannon House.
He interested himself in the economic development of Ireland and was the author of a pamphlet, Essay on commerce, published in 1777.
The three eldest sons died young, while the fourth was John FitzGibbon, who was also a lawyer and was made Earl of Clare in 1795.