Armstrong, John Warneford (1770–1858), soldier and informant, was born 28 August 1770 at Ballycumber, near Clara, King's Co. (Offaly), eldest among two sons and three daughters of George Armstrong (1734–80), landed gentleman, of Ballycumber, and his wife Constantia Maria (d. 1826), eldest daughter of Andrew Armstrong of Gallen, King's Co. After some patchy schooling in England (1779–80), Dublin, and Portarlington, he received a good education when, aged 15, he enrolled in the school of Samuel Whyte (qv) in Grafton St., Dublin. His father's estate was heavily encumbered and on inheriting it in 1791 John fled to Portugal to avoid legal proceedings. He moved between Portugal, Dublin, and London until becoming ensign in the Somerset militia (27 April 1794). Stationed at Dover, he was promoted lieutenant 19 June 1795. He quit the militia in late 1796 and returned to Ireland, engaging regularly in amateur dramatics. Appointed captain-lieutenant in the King's Co. militia on 19 March 1798, he was based at Loughlinstown near Dublin.
An avid reader, he regularly visited the bookshop of Patrick Byrne (qv) in Grafton St. to buy radical pamphlets and discuss politics, often expressing strong democratic views. On 10 May 1798 Byrne, a United Irishman, introduced him to United leaders John (qv) and Henry Sheares (qv) as ‘a true brother’ (TCD MS 6409, f. 56). The Sheares, believing disaffection to be rife in the King's Co. militia, asked if he could bring the regiment over to the United Irishmen. Armstrong indicated his willingness to help, but immediately afterwards reported the approach to his commanding officer Col. Henry L'Estrange, who advised him to feign interest. He met the Sheares several times at their home in Baggot Street (13–20 May), and on 17 May they introduced him to William Lawless (qv) and discussed seizing the Loughlinstown camp. Armstrong reported everything back to L'Estrange and Lord Castlereagh (qv), and the Sheares were arrested on 21 May.
After the rising broke out on 23 May, he went to Kildare to join his regiment (30 May), but failed to find them and was given command of a Londonderry militia company. He took part in an attack on rebels near Blessington but after being shot in the foot in a fierce fight near Slievebuy mountain outside Gorey (4 June) returned to Dublin for treatment. On 12 July he testified in court against the Sheares. The defence, led by J. P. Curran (qv), produced several witnesses who claimed that Armstrong was an atheist and a Paineite, who had quit the Somerset militia because of his democratic principles and had boasted of his willingness to act as George III's executioner. But Armstrong bore up well under Curran's cross-examination and at times got the better of him:
‘Armstrong: Going up Blackmore Hill . . . we met three men with green cockades: one we shot, another we hanged, and the third we flogged and made a guide of.
Curran: Which did you make the guide of?
Armstrong: The one that was neither shot nor hanged’ (Madden (1860), 359).
Curran focused on the callousness of Armstrong's character, painting a vivid picture of him dining at the Sheares' table, conversing pleasantly with their family, and even caressing Henry's children, knowing all the time that he was leading his hosts to the gallows. Armstrong's evidence did indeed convict both Sheares, who were hanged on 14 July. He also testified against Lawless before a lords' special committee in August.
Lauded by loyalists for saving Dublin from massacre, Armstrong was granted the freedom of the city by Dublin corporation (20 July 1798). On 2 August he was given command of troops around Newtownmountkennedy in North Wicklow, and ordered his men to shoot suspected rebels on sight, even those with official pardons. This led to a much publicised case when the viceroy, Lord Cornwallis (qv), publicly rebuked the officers of a court martial that had acquitted Hugh Woolaghan, a yeoman who had deliberately killed a pardoned rebel; Woolaghan claimed he was simply carrying out Armstrong's orders. On 23 December 1798 King's Co. militia officers presented Armstrong with a gold medal praising his conduct in convicting the Sheares, and in December 1799 he received a government pension of £500 a year.
He remained with the King's Co. militia until 28 June 1801, resigning in frustration at junior officers being promoted over his head. On 20 August 1802 he was appointed JP for King's Co. and on 7 September captain of the Castle Armstrong yeomanry. He became ensign in the 66th Regiment (30 January 1806), based in Clonmel, and bought a lieutenancy (20 August 1806), transferring as lieutenant to the 1st Foot (7 May 1807) and the 14th Foot (12 December 1807), and served briefly in Spain (October 1808–February 1809); on 14 January 1810 he went on half pay. He toured France (1815–17, 1822–3), Switzerland (1817–18), and Italy (1818–22), and returned to Ballycumber in June 1823 to live comfortably on his pension and rents of £200 a year. Active in local politics, he supported his first cousin Sir Andrew Armstrong (1786–1863), whig MP for King's Co. (1841–52), and attended to his duties as JP and grand juryman of King's Co. He was generally on good terms with his tenants and neighbours, but had a reputation as a severe magistrate.
After R. R. Madden (qv) published United Irishmen (1843), Armstrong wrote to him offering to correct some errors. They met 6 October 1843 and Madden found him courteous but unrepentant. Armstrong claimed that he had only done his duty in 1798, since he believed that the United Irishmen would plunge the country into anarchy and sectarian slaughter. He had taken no oath or test and had only dined once with the Sheares (on Castlereagh's suggestion) — an act he regretted. He denied caressing Henry Sheares's children, maintaining that he was not particularly fond of children and rarely paid them any attention. Nationalists never forgave him: his face was slashed and badly scarred by a rebel sympathiser and in 1843 the Nation was scandalised that he was living openly at Ballycumber, making no attempt to hide his past deeds, and noted that the informer Thomas Reynolds (qv) ‘was an angel to this man’ (23 September 1843).
Armstrong retained his health and vitality into old age – even in his eighties he attended petty sessions at Ballycumber twice weekly and travelled regularly to Dublin to draw his pension. He died 20 April 1858 at Ballycumber, and was buried in the family vault at Liss church, Ballycumber. His papers are held in TCD.
He married (18 October 1806) Anne, daughter of William Turner of Gloucester, and had two daughters, Mary and Anne (d. 1878). His brother Andrew George Armstrong (1773–1821) was captain in the 104th Regiment.