Mystery surrounds Lawless's involvement in the 1798 Rebellion and the Irish rebellion of 1803, which were designed to establish an independent republic in Ireland. He has been cited as the chief organiser of the United Irish Movement in London, but downplayed this aspect of his life in his later writings when the democracy movement had long been suppressed. He is believed to have joined the United Irishmen in 1793, shortly before his father, the first Lord Cloncurry, took charge of Lyons House. Valentine was imprisoned in June 1798 on suspicion of treason in London, released, re-arrested and held in the Tower of London until March 1801. It was widely believed that his long imprisonment hastened his father's death in August 1799. Lawless’s agent Thomas Braughall was also arrested and he was asked to subscribe to the defence of Father James O'Coigly, a United Irish leader hanged in London in 1798.