The South Longford by-election of 1917 was held on 9 May 1917 due to the death of the incumbent Irish Parliamentary MP, John Phillips. The by-election ended in a surprise Sinn Féin victory over the Irish Parliamentary Party following a very close vote. The result was not announced until 10 May due to a recount. The result of the by-election increased pressure on the British Government to free the remaining Irish prisoners in Lewes. They were eventually freed in June 1917. McGuinness subsequently sat for the Longford Constituency in the Dáil Éireann.