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 would also have being seen as a threat to George III, the current monarch, who's Coronation Oath held him to uphold and secure the Protestant faith. Article Fifth of the Act of Union combined the Church of Ireland and the Church of England into one central Protestant Episcopal Church. It also made the protestant faith the official religion of Ireland. The unity of the churches would also have bred the hope that more of the Catholic majority in Ireland would be converted. This would have being a vital cause for the union getting voted through the all protestant Irish parliament, as the protestants were outnumbered by Catholics in the general population at a ratio of 3:11. Should the Act of Union pass, they would be on the opposite side, it would be a 3:11 protestant majority.
On a related topic, Robert Peel had earlier being responsible for the ending of several of the penal laws, all since the threat of war began in France. In 1793 Catholics could vote and become lawyers for the first time. He also played a part in the repeal of Poynings Law, which allowed the Irish parliament to enact its own laws without influence from London for the first time since the Norman Invasion. However, vitally, he could not hold his nerve to repeal the Penal law that stopped Catholics from holding a place in parliament and many Irish people would have thought this was the most important, as they could only vote for the entirely protestant government. Pitt had supported the Act Of Union in 1800, but had originally planned to follow it with more far reaching ideas, such as Catholic emancipation. However George III, after signing the Act of Union into law in August 1800, refused to support full Catholic emancipation on the basis that it would be contrary to his Coronation Oath.
While the Act of Union was defeated the first time in the Irish parliament, it was passed on the second time of asking. Certain peerages, pensions and certain honors were offered to Irish politicians and...