Born at Oldcastle, County Meath, on November 20, 1829, Halpine is the son of the Rev. Nicholas John Halpin. He is educated at Trinity College, Dublin, until 1846, with original intentions for the medical profession, but ultimately prefers the law. In his leisure time he writes for the press. The sudden death of his father and his own early marriage compel him to adopt journalism as a profession.
In 1851 he emigrates to the United States, and takes up residence in Boston, where he becomes assistant editor of The Boston Post, and, with Benjamin Penhallow Shillaber, commences a humorous journal called The Carpet Bag, which is unsuccessful. Afterwards he resides in Washington, D.C., where he acts as the correspondent of The New York Times.
After moving back to New York he secures employment with the New York Herald, and in a few months establishes relations with several periodicals. He undertakes a great variety of literary work, most of which is entirely ephemeral. He next becomes associate editor of The New York Times, for which he writes in 1855 and 1856 the Nicaragua correspondence at the time of William Walker‘s filibustering expedition. In 1857 he becomes principal editor and part proprietor of the New York Leader, which under his management rapidly increases in circulation.
At the beginning of the American Civil War in April 1861, he enlists in the 69th New York Infantry, in which he is soon elected a lieutenant, and serves during the three months for which he has volunteered. He is then transferred to the staff of General David Hunter as assistant-adjutant-general with the rank of major, and soon afterwards goes with him to Missouri to relieve General John Charles Frémont. He accompanies General Hunter to Hilton Head, and while there writes a series of burlesque poems in the assumed character of an Irish private. Several of them are contributed to the New York Herald in 1862 under the pseudonym of “Miles O’Reilly,” and with additional articles are issued in two volumes entitled Life and Adventures, Songs, Services, and Speeches of Private Miles O’Reilly, 47th Regiment New York Volunteers (1864), and Baked Meats of the Funeral, a Collection of Essays, Poems, Speeches, and Banquets, by Private Miles O’Reilly, late of the 47th Regiment New York Volunteer Infantry, 10th Army Corps. Collected, revised, and edited, with the requisite corrections of punctuation, spelling, and grammar, by an Ex-Colonel of the Adjutant-General’s Department, with whom the Private formerly served as Lance-Corporal of Orderlies (1866).
Halpine is subsequently assistant-adjutant-general on General Henry W. Halleck‘s staff with the rank of colonel in 1862, and accompanies General David Hunter as a staff-officer on his expedition up the Shenandoah Valley in the spring of 1864. On his return to New York he resigns his commission in consequence of his bad eyesight, receiving the brevet of brigadier-general of volunteers.
He then makes New York his home and resumes his literary work. He becomes editor and, later, proprietor of the Citizen, a newspaper issued by the citizens’ association to advocate reforms in the civil administration of New York City. In 1867 he is elected registrar of the county of New York by a coalition of republicans and democrats. Incessant labour brings on insomnia. He has recourse to opiates, and his death in New York City on August 3, 1868 is caused by an undiluted dose of chloroform.