In the 1890s she became interested in Irish history and the nationalist movement as a result of her friendship with John Francis Taylor. She was vocal in her opposition to English colonial policy in South Africa during the Boer Wars and supported Roger Casement's Congo Reform movement. Her 1908 book The Making of Ireland and its Undoing argued for the sophistication and richness of the native Irish civilisation. Stopford Green was active in efforts to make the prospect of Home Rule more palatable to Ulster Unionists. She was closely involved in the Howth gun-running.