Larcom, Sir Thomas Aiskew (1801–79), surveyor, administrator, and under-secretary for Ireland (1853–68), was born 22 April 1801 in Hampshire, one of five children of Captain Joseph Larcom (d. 1817), RN, who administered a dockyard in Malta, and his wife Ann (née Liverstoke). After studying at the Royal Military Academy at Woolwich, he was commissioned second lieutenant in the Royal Engineers in June 1820 and served for two years in Gibraltar. In 1824 he joined the Ordnance Survey of England, directed by Thomas Colby (qv). Promoted to lieutenant, Larcom was sent to Ireland in 1826 to work on the Irish survey, and in 1828 was appointed Colby's assistant in the headquarters of the Irish survey at Mountjoy House, Phoenix Park, Dublin. Larcom was put in charge of the office for engraving maps, but since he was energetic, widely read, systematic, and orderly, soon came to dominate the whole business of ordnance surveying. He oversaw the division of the various personnel needed for map-making – compilers, draughtsmen, engravers, printers, and examiners – into separate branches and sub-branches, and effectively coordinated their activities. The entire project was marked by his vigour, dedication, and attention to detail. Concerned that place names were becoming confused, he first took Irish lessons himself, and then appointed his young teacher, John O'Donovan (qv) to the Ordnance Survey. He also employed George Petrie (qv) and Eugene O'Curry (qv) on the survey, giving an important boost to Irish antiquarian studies. Larcom was elected MRIA in 1833.
An early advocate of the continental practice of contouring, Larcom also introduced to Ireland the electrotype process, which facilitated the updating of maps. Under his direction, Mountjoy became known as a centre of innovation. Larcom was involved in all the map-making enterprises required by an increasingly reformist Irish administration: he prepared the new maps required by the Irish parliamentary reform act of 1832, compiled a map on which proposed Irish railway routes were shown, and completed detailed maps of sixty-seven towns for the Report on Irish municipal reform (1836). He also established a meteorological observatory in the Phoenix Park, intending it to form part of a national network of weather stations. An exacting and strict boss, he often referred to his men as machines, and once reprimanded the lively and humorous O'Donovan for using ribald language in official correspondence. Larcom was, however, much admired by his staff for his ability and fairness, and inspired great loyalty.
After the success of the Ordnance Survey's 6 inch maps in the 1830s, Larcom sought to extend the project's work to include detailed memoirs of each locality. He envisaged a complete regional description, comprising history, archaeology, geology, topography, living conditions, and economics. The Irish administration approved but the publication of the first memoir of Templemore in Co. Londonderry in 1835 provoked controversy. The cost was considerable and the under-secretary, Thomas Drummond (qv), was wary of proceeding further, while the chancellor of the exchequer, Thomas Spring Rice (qv), thought the scheme overly dependent on Larcom's abilities and interests. Learned societies and much of the press, however, were supportive, praising the memoir as an important contribution to the economic, historical, and geographical knowledge of Ireland, and Larcom was championed by a wide circle of influential friends. A commission appointed in 1843 to consider the scheme was inconclusive, and the prime minister, Sir Robert Peel (qv), demanded that the cost of memoirs be met by private subscription. Larcom received little backing from his superiors: Colby's support for the memoirs was decidedly half-hearted, while the inspector-general of the survey, Sir Frederick Mulcaster, believed they were excessively detailed and the entire scheme too expensive. No further memoirs were published at the time.
In 1841 Larcom was appointed a census commissioner. Believing that comprehensive and accurate statistical data were the basis for good government, he introduced systematic methods of collecting and classifying census information, and was a founder member of the Dublin Statistical Society in 1847. At his instigation, the Irish census of 1841 included a classification of occupations and the general condition of the population, and he established a permanent branch of the registrar-general's department to collect agricultural statistics; thematic maps accompanied all census reports. Several of his innovations and methods were later adopted by the English census. He was much in demand as commissioner of various inquiries, such as that into the queen's colleges in 1845.
By early 1846 simmering tensions between Larcom and Colby came to a head. Colby was increasingly jealous of Larcom's independence and of his stature in Ireland – the 6 inch maps were known colloquially as ‘Larcom's maps’. Forced by the treasury to make savings, Colby began to assert his authority over the Irish Ordnance Survey and was resisted by Larcom. On 17 March 1846 Larcom was appointed to the famine relief commission to examine the feasibility of various relief schemes, and to assist him in this work, held onto survey records requested by Colby. Colby responded by dismissing Larcom on 4 May 1848; he had to face down opposition from Peel and the chief secretary, Lord Lincoln (qv), but in the administrative confusion that followed the collapse of Peel's ministry in June, the decision stood. With an excellent reputation for integrity and efficiency, Larcom was much sought after for official positions, and was appointed commissioner of the board of works (1846–9), where he used his detailed knowledge of the country to organise public relief works such as road building and land drainage. The intense effort he devoted to relief projects took a heavy toll on his health. In 1849 he sat on the commission to investigate the reform of Dublin corporation, and the following year was elected to the senate of the Queen's University. He served as deputy chairman of the board of works (1850–53) and during these years prepared a well-regarded edition of William Petty's (qv) Down Survey (1851).
In January 1853 Larcom was appointed under-secretary for Ireland under Aberdeen's liberal-Peelite administration. He was the first permanent holder of the office, and during a period of political instability provided a valuable level of continuity and expertise under six successive lords lieutenant until his retirement in 1868. Energy and efficiency – attested to by his well-organised archives – were the hallmarks of his sixteen-year tenure. More a pragmatic reformer than a radical innovator, he implemented significant improvements in local taxation, the poor law, medical dispensaries, and local government. He drew up detailed memoranda for his superiors, which showed steady improvements in agricultural output and living standards, and reduced levels of crime. Like his friend Drummond, he believed that Ireland needed firm but fair government, free from any sectarian or party political bias. (Larcom himself was always guarded in expressing political opinions.) After receiving reports of police involvement in sectarian rioting in Belfast in August 1864, he recommended placing the city under police magistrates, banning all processions, and proscribing Orange societies. Paying little heed to critics in parliament or the press, he readily employed the full powers of the Irish administration. During an outbreak of foot and mouth disease, for example, he adopted the ‘Prussian’ policy of using the constabulary to enforce the slaughtering of sick animals and the isolating of infected areas, and his efforts had significantly more success than those of the authorities in England and Scotland. He was promoted to major-general in April 1858, and created KCB in June 1860. His pervasive influence on the Irish administration was such that it was often said, regardless of who was in power, that Ireland was governed by ‘Larcom and the police’.
The biggest challenge he faced was the growth of Fenianism. At the height of the threat in the mid 1860s, the chief secretary, Lord Naas (qv), was mostly resident in London, and relied largely on Larcom's handling of events. Recognising that the movement was nourished by the grievances of over a million expatriate Irishmen and had the potential to become a formidable threat, Larcom directed the arrest of IRB leaders and the suppression of the Irish People in September 1865. He left nothing to chance and pressed for the suspension of habeas corpus in February 1866 (and for its continued suspension until 1869), and for increased numbers of troops to be deployed in Ireland. The failure of the commander-in-chief, Lord Strathnairn, to move troops from the Curragh to potential trouble spots exasperated him, and he observed that Strathnairn cared more about the danger to his men than the danger to the country. Larcom sought to scotch the threat of Fenianism by gathering as much information as possible to forestall any insurrectionary attempts. When rebellion finally broke out in March 1867, he adopted a firm but measured response, and was anxious to avoid stirring up the country by executions or heavy-handed coercion. He refused, for example, the calls to ban the massive funeral demonstrations for the Manchester Martyrs held in December 1867, but instead kept them under close surveillance. However, personal circumstances threatened to embarrass him: after the death of John O'Donovan in 1861, Larcom had been named guardian of his sons, several of whom were Fenians. In deference to O'Donovan's memory, Larcom intervened on a number of occasions to have them released from prison, and this apparently was common knowledge in Dublin.
In his final years as under-secretary, Larcom found his duties increasingly onerous, and was prone to poor health and fainting fits. He regularly asked to be relieved of his position, but was considered indispensable to the Irish administration and was persuaded to remain in office. He finally retired in December 1868. On the recommendation of Naas (then Lord Mayo), he was created baronet (28 December 1868) and appointed to the Irish privy council. Larcom spent much of his retirement collating information about Ireland during his term of office, which he arranged in volumes and presented to various Irish learned societies. He died 15 June 1879 at Heathfield, near Fareham, Hampshire, and was survived by his wife Georgina, daughter of General Sir George d'Aguilar (m. 1 March 1840). Of their five children, the first two sons predeceased him and he was succeeded as 2nd baronet by his third son, Lt-Col. Sir Charles Larcom (1843–92).
Larcom's extensive papers are held in the NLI and TCD in hundreds of uniformly bound volumes (many consisting of newspaper cuttings).