At the same time, a new organisation was formed in Ireland. Under Wolfe Tone, the United Irishmen (who consisted of Protestants and Catholics alike) declared their belief in a peaceful future for Ireland in which Protestants and Catholics could live together in peace and with equality. They wanted to set up a French-styled democratic republic in Ireland, which was independent of Britain. They quickly gained support, although some, most notably the newly-formed Orange Order which was set up to preserve loyalty to the monarchy, were against them.

Supporting French Republicanism was seen as treasonous by the British considering they were at war with France. Also, Britain was a constitutional monarchy, which meant that the King did not have absolute power. Therefore the British regarded themselves as already democratic. Because of these facts, the British saw the United Irishmen as a national threat to be disposed of.

In 1798, the British began attacking known United Irishmen, and murdering large numbers of Protestant and Catholic members. Tone realised that if they were going to have their rebellion, it would have to be now or never, before the British destroyed them. So a large rebellion began in the spring concentrated in counties Down, Antrim and Wexford. Several bloody battles took place at Antrim, Ballynahinch and Saintfield. The United Irishmen were finally defeated at the Battle of Vinegar Hill in County Wexford. Almost all the several hundred United Irishmen were slaughtered.

However, it was not over. In late 1798, the French sent reinforcements to Ireland and they landed at Mayo, in western Ireland. They invaded and took over the area and gained popular support among the local Irish who saw it as an opportunity to get a better government. The French and their Irish allies got as far as county Sligo before being defeated by the British. While the French were taken prisoner, the local Irish were massacred as a punishment for treason. Wolfe Tone committed suicide in prison whilst awaiting execution. Note that this was the last time a hostile army ever invaded Ireland.

Although the rebellion had been put down, it was clear that Republicanism in Ireland could not be ignored and serious changes were needed in the way Ireland was governed to ensure that such violence did not occur again.