The Society of United Irishmen was founded in Belfast in 1791. Its inspiration was a young Dublin lawyer, Theobald Wolfe Tone, who was invited to Ulster after publishing a pamphlet entitled "An argument on behalf of the Catholics of Ireland". Northern Presbyterians also suffered from religious discrimination, though less severely, and had absorbed republican ideas from the American and French revolutions. With the formation of a Dublin society, pressure for reform grew, and relief acts were passed in 1792 and 1793. However, Tone sought revolution rather than reform, and hoped for French help in severing the link with Great Britain. After Britain and France went to war in 1793, the United Irishmen came under increasing pressure from the government. Tone chose exile in America in preference to being prosecuted for treason, and the United Irishmen evolved into a secret society bound by revolutionary oaths.

Returning to Europe in 1796, Tone persuaded the French to invade Ireland, but bad weather prevented a landing. Despite this setback, the United Irishmen continued to recruit members, particularly among disaffected Catholic peasants. Meanwhile, the government had passed an act providing for harsh measures against those who held illegal arms or administered illegal oaths. An army under General Lake conducted an oppressive campaign to disarm Ulster, seen as the most dangerous province. The government had many informers among the United Irishmen, and in March 1798 most of the Leinster leaders were arrested in Dublin. The only leader of the United Inshmen with military experience, Lord Edward Fitzgerald, was captured on 19 May, four days before the date fixed for the rising.