In 1769 he was called to the Irish Bar, and he soon obtained a large practice, and won a great reputation both as a sound lawyer and an eloquent pleader.
He supported all the motions of the radical parties, and in 1782 he made his most famous speech in proposing a certain measure of Catholic relief.
Fitzgerald never sought political office, but he eagerly accepted professional appointments, which helped him at the bar.
He opposed the project of the Union with all his might, and he was certainly disadvantaged in his cause, for in 1799 he was dismissed from his post of Prime Serjeant to make way for St George Daly, who had been converted to the Unionist policy.
The Irish bar insisted on showing their respect for him, and continued to give him precedence in court over the Attorney-General and Solicitor-General which he had held as Prime Serjeant.