There were several significant causes of the Act of Union between Great Britain and Ireland in 1800, most notably, the United Irishmen rebellion of 1798, along with the French landing at Killala in North Mayo. The United Irishmen, a radical mixed religious group, had began a campaign against British rule in Ireland in 1798. This rebellion was centered around Wexford, Wicklow and a protestant linen worker rebellion in Antrim. The rebellion was poorly organized and coordinated, and many parts of the country were left undisturbed. Although it was yet another rebellion by the Irish that was defeated, it fast forwarded the long standing idea that a political, constitutional and military union was needed between the two countries to prevent further war, or even worse, for Ireland to become a stepping stone in a French invasion of The British Isles. The worry about a French invasion starting in Ireland was compounded by a small French landing in north Mayo, that led to two battles, at Castlebar and Ballinamuck. The French landing was requested by Wolfe Tone, a protestant who was viewed as the leader of the United Irishmen. Wolfe Tone was influenced by the French and American Revolutions, and passed this influence onto the United Irishmen.
The fact that the American Revolution had occurred so recently also had another major bearing on the Act of Union. the British parliament did not want to lose another colony, especially not one this close to home. This most likely would have being seen as a major weakness by the other European powers of the time.
The French revolution, which promised freedom to all religions and races, and equal rights to all men
Bibliography: Brown, Micheal, James Kelly, Patrick M.Geoghegan, The Irish Act of Union,1800, (Irish Academic Press, 2003) Dr McGrath Lecture Slides, Contested Island Lalor, Brian (ed). The Encyclopaedia of Ireland. Gill & Macmillan, Dublin, Ireland, 2003. Ward, Alan J. The Irish Constitutional Tradition, (Irish Academic Press, 1994) http://www.bbc.co.uk/history/british/empire_seapower/acts_of_union_01.shtml 28th March 2012 [ 9 ]. Alan J. Ward, The Irish Constitutional Tradition p.28(Irish Academic Press, 1994) [ 10 ]