Fuelled by the rhetoric of French Republicanism, Irishmen joined forces under the United Irishmen to rebel against the English supremacy over Ireland. The organisation contained both Catholics and Protestants (mainly Presbyterians who found common cause with the Catholics), but the emphasis by some on the need for Catholic Emancipation had caused some Protestants to leave the ranks. The rebellion, when it did come in 1798, was not as well supported as the United Irishmen had hoped. They did not receive the intended support from the French, although French colours were captured by the Armagh militia in County Longford.

The rebellion had little impact on the area around Markethill. Given the hardships of the time, it is likely few of the residents of this locality would have noticed any significant change in their daily lives. Two years later in 1800, the Act of Union was passed. In 1806, Sir Arthur Acheson received an Earldom for his loyalty during the insurrection and for his support for the Act of Union.