Thomas Addis Emmet (1764-1827) was the older brother of Robert Emmet. He initially qualified as a physician, but decided to study law after the sudden death of his older brother, Christopher Temple Emmet, who was a barrister of great distinction. From 1790, Addis Emmet practiced as counsel for those charged with political offences, and was the leading advocate for the United Irishmen.

In 1795, he took the oath of the Society of United Irishmen in open court. Soon after this, he became secretary of the organisation and a member of its executive. After the 1798 Rebellion, he was imprisoned at Fort George until 1802. He was in Paris when he heard the news of his brother Robert Emmet's capture, and decided to emigrate to the United States. He obtained a thriving legal practice in New York, and was Attorney General of that city in 1812-13. He was said by the famous judge, Joseph Story, to be 'the favourite counselor of New York'.