He was called to the Irish bar in 1731, and in November of the same year he converted to the established Church of Ireland. He also published a legal textbook, Notes on cases determined by Westminster, in 1731. Fitzgibbon amassed a considerable personal fortune practising as a barrister and consulting lawyer, with which he purchased Mountshannon House. Despite his conversion, he remained sympathetic to Irish Catholics and in March 1767 Fitzgibbon defended in court Catholics accused of Whiteboy crimes and treason.[1]
In 1761, FitzGibbon was elected as a Member of Parliament for Newcastle in the Irish House of Commons; in 1768 he was elected to sit for Jamestown.[2] At first he supported the government but by 1763 he had joined the opposition, where he remained until he retired from parliament in 1776. He interested himself in the economic development of Ireland and was the author of a pamphlet, Essay on commerce, published in 1777.[1]