Sinn Féin (which translates in English as “we ourselves”) was founded as an Irish nationalist political party in 1905 by Arthur Griffith. The party initially held to the dual monarchy system which Griffith famously proposed in his work The Resurrection of Hungary, however adopted republicanism following the Easter Rising. The party also adopted a formal policy of abstentionism, refusing to recognise or take their seats in the British Parliament. In 1918, Sinn Féin would win 73 out of 105 seats in the Irish general election and would help to establish the Dáil Éireann in early 1919. During the Civil War, pro-Treaty TDs split from the party to form Cumann na nGaedheal, and Sinn Féin, which was staunchly anti-Treaty, would remain a strictly abstentionist party refusing to recognise either Westminster or Leinster House. The party would undergo further splits, including the Official-Provisional split of the late 60s, and the formation of Republican Sinn Féin in 1986 following the removal of the policy of abstentionism to Leinster House. The current Sinn Féin, which has its roots in Provisional Sinn Féin, is currently one of the largest parties both north and south of the border.
The Sinn Féin Manifesto (1918)