Rebellion was in the air throughout Ireland as the 18th century came to a close. Penal Laws discriminated against both the majority Irish Catholic population and non-Anglican Protestants. A small group of liberal Protestant in Belfast founded the Society of United Irishmen in 1791. The organisation crossed the religious divide with membership comprising Roman Catholics, Presbyterians, Methodists, other Protestant "dissenter" groups. The British Government became alarmed at this and promoted animosity between Orangemen and the United Irishmen, resulting in the establishment of the Orange Order in 1795. Support for the United Irishmen spread throughout Ireland and by 1797, it had 200,000 members. In March 1798, the plague of "informants" resulted in the arrest of much of the United Irishmen leadership. Martial law was imposed over most of the country and its unrelenting brutality put the United Irish organisation under severe pressure to act before it was too late.
The first clashes of the rebellion took place on 24 May. Fighting quickly spread throughout Leinster, with the heaviest fighting taking place in County Kildare where rebels gained control of much of the county. However, rebel defeats at Carlow and County Meath ended the rebellion in those counties. In County Wicklow, news of the rising spread panic and fear among loyalists who massacred rebel suspects at Dunlavin.
In the north-east, mostly Presbyterian rebels led by Henry Joy McCracken rose in County Antrim on 6 June. They briefly held most of the county, but the rising there collapsed following defeat at Antrim town. In County Down, after initial success at Saintfield, rebels led by Henry Munro were defeated at Ballynahinch.
The rebels had most success in Wexford where they seized control of the county, but defeats at New Ross, Bunclody and Vinegar Hill led to final defeats on 14 July at Knightstown Bog, County Meath and Ballyboughal, County Dublin.
On 22 August, nearly two months after the main uprisings had been defeated, about 1,000 French soldiers under General Humbert landed in County Mayo. Joined by up to 5,000 local rebels, they had some initial success, inflicting a humiliating defeat on the British at the Castlebar. Supportive risings in Longford and Westmeath were quickly put down on 8 September 1798. The French troops who surrendered were repatriated to France in exchange for British prisoners of war, but hundreds of the captured Irish rebels were executed.
Casulty estimates varying from 20,000 to 50,000 deaths are probably too high. However, the 1798 rebellion was the most bloody event in Irish history.