Halpin, William G. (1824–92), Fenian, was born in May 1824 in Nobber, Co. Meath, son of John Halpin and his wife Sally. After studying civil engineering in Dublin, he left for the USA, arriving in New York on 28 October 1847. Shortly thereafter he settled in Cincinnati, Ohio, where he became a prosperous county surveyor and civil engineer. He married (date unknown) Catherine Lynch (d. 1850) and had one son, William (d. 1873). His brothers, John and Anthony, and his sister Mary, followed him to the US in the early 1850s. Around this time he wrote some verse which was later reprinted in the Nation and other Irish newspapers. In September 1855 he became the president of the Irish Emigrant Aid Society of Ohio, a body that secretly supported armed revolution in Ireland. Previously he had been a member of a similar organisation known as the Exiled Sons of Ireland. In January 1856, after the intervention of the British consulate in Washington, he was tried by the district court of Ohio for violation of the neutrality laws but was found not guilty. He joined the Fenian Brotherhood shortly after it was established in 1858 and organised it in Ohio. On the outbreak of the American civil war (April 1861) he became a captain in the Fifteenth Kentucky Infantry and fought for the union. He was promoted to major on 13 July 1863 and to lieutenant-colonel on 26 January 1864. Between September 1863 and January 1865 he commanded the regiment in many of its battles, including the battle of Chickamauga and the siege of Atlanta.
In early 1865 he left for Ireland as an official representative of the Fenian Brotherhood and sat on the military council of the IRB. Despite the arrests that followed the suppression of the Irish People in September 1865, he remained confident of the chances of a rising. He returned to the US in the spring of 1866 and was appointed by John O'Mahony (qv) as chief mediator in an attempt to reunite the Fenian Brotherhood. He supported the deposition of James Stephens (qv) as leader of the IRB (15 December 1866) and was appointed second-in-command under Col. Thomas Kelly (qv). On 12 January 1867 he left for England and set about planning a rising in Ireland. In February 1867, while in London, he helped to draft a proclamation of an Irish republic. Thereafter he was given the responsibility of organising a rising in Dublin, but soon discovered that the Dublin IRB had little or no arms. On 5 March he was stationed at Killakee in the Dublin mountains with about sixty men, awaiting the arrival of IRB troops who were to march out from the city. These, however, were dispersed by the RIC at Stepaside and Tallaght, and the plan had to be abandoned.
After spending several months in hiding, Halpin went to Liverpool and boarded a ship to New York. However, on 4 July the ship was stopped off the coast of Cork by the Royal Navy and he was arrested. During his trial in Dublin he defended himself unsuccessfully in court. On 16 November 1867 he was sentenced to fifteen years’ penal servitude, and was sent to Millbank and subsequently to Chatham prison. In December 1870 he was offered his release on condition that he would leave Ireland and never return, which he refused to accept. In January 1871 the American government intervened and secured his unconditional release. He was fêted on his return to Cincinnati on 17 March 1871 and was nominated for mayor, but declined. He supported the establishment of the short-lived Irish Confederation in New York (1871–3). In 1875 he was appointed chief engineer of Cincinnati. In the spring of 1877, in an attempt to revive the Fenian Brotherhood, he published his own newspaper, the Irish Vindicator, which stayed in existence until September 1879. During the 1880s he did not take a prominent part in Irish revolutionary affairs, and he died 8 May 1892 at his nephew's home in Cincinnati, Ohio.