Final answer:
After negotiating the 1921 treaty with the British, Arthur Griffith founded the political party Sinn Féin.
Step-by-step explanation:
After negotiating the 1921 treaty with the British, Arthur Griffith founded the political party Sinn Féin. Sinn Féin, which means 'We Ourselves' in Irish, was originally organized in 1905 to fight for an independent Ireland and was often associated with the Irish Republican Army (IRA). While the Irish Nationalist Party also fought for an independent Ireland, Sinn Féin and the IRA sometimes adopted more violent methods.