A Dublin Protestant and the son of an ironmonger, Tandy is baptised in St. Audoen’s Church on February 16, 1739. He attends the Quaker boarding school in Ballitore, County Kildare. He starts life as a small tradesman. Turning to politics, he becomes a member of Dublin Corporation and is popular for his denunciation of municipal corruption and his proposal of a boycott of English goods in Ireland in retaliation for the restrictions imposed by the government on Irish commerce.
Tandy and John Binns persuade Dublin Corporation to condemn by resolution William Pitt the Younger‘s amended commercial resolutions in 1785. He becomes a member of the Whig club founded by Henry Grattan, and he actively co-operates with Theobald Wolfe Tone in founding the Society of United Irishmen in 1791, of which he becomes the first secretary.
Sympathy with the French Revolution is rapidly spreading in Ireland. A meeting of some 6,000 people in Belfast vote a congratulatory address to the French nation in July 1791. In the following year, Tandy takes a leading part in organising a new military association in Ireland modelled after the French National Guard. Tandy also, with the purpose of bringing about a fusion between the Defenders and the United Irishmen, took the oath of the Defenders, a Roman Catholic society whose agrarian and political violence had been increasing for several years.
Tandy is about to be tried in 1793 for distributing a seditious pamphlet in County Louth when the government discovers he has taken the oath of the Defenders. Being threatened with prosecution for this step, and also for libel, he takes refuge by changing his Dublin address often until he flees to the United States in 1795, where he remains until 1798. In February 1798 he goes to Paris, where a number of Irish refugees are assembled and planning rebellion in Ireland to be supported by a French invasion, but quarrelling among themselves over tactics.
Tandy accepts the offer of a corvette, the HMS Anacreon, from the French government and sails from Dunkirk accompanied by a few United Irishmen, a small force of men and a considerable quantity of arms and ammunition for distribution in Ireland. He arrives at the isle of Arranmore, off the coast of County Donegal, on September 16, 1798.
Tandy takes possession of the village of Rutland, where he hoists an Irish flag and issues a proclamation. He soon discovers that the French expedition of General Jean Joseph Amable Humbert to aid the Irish rebellion has failed. He sails his vessel around the north of Scotland to avoid the British fleet. He reaches Bergen in safety having brought with him a British ship captured along the way. Tandy then made his way with three or four companions to the free port of Hamburg but a peremptory demand from the British government to detain the fugitives was acceded to despite a counter-threat from the French Directory. In 1799 HMS Xenophon, under Commander George Sayer, brings Tandy and some of his associates back to England as state prisoners.
On February 12, 1800, Tandy is put on trial at Dublin and is acquitted. He remains in prison in Lifford Gaol in County Donegal until April 1801, when he is tried for the treasonable landing on Rutland Island. He pleads guilty and is sentenced to death although he is reprieved and allowed to go to France.
In France, where his release is regarded as a French diplomatic victory, he is received, in March 1802, as a person of distinction. When he dies on August 24, 1803 in Bordeaux, his funeral is attended by the military and an immense number of civilians. James Napper Tandy is buried in his family’s burial crypt, St. Mary’s churchyard, Julianstown, County Meath. His fame is perpetuated in the Irish ballad The Wearing of the Green.