What the 1851 census Irish emigration record measured

The 1851 census Irish emigration record begins with one number: a population fall of 1,622,739 in ten years, from 8,175,124 in 1841 to 6,552,385 in 1851. It was the first Irish census taken after the Great Famine, and its returns on births, deaths, and housing gave the government its first hard measure of the loss.

That fall was not a single event. It was deaths, unborn children, and people who left, all folded into one number by census enumerators who had no way of separating the three with certainty. Untangling that number is still the work of Irish demographic history.

1851 census Irish emigration numbers: the national picture

The census was taken on the night of 30 March 1851, under the superintendence of the Registrar-General of Marriages, William Donnelly, with the Dublin surgeon William Wilde acting as medical census commissioner. Thomas Larcom, who had led the 1841 count, had been the intended chief commissioner for 1851 but withdrew because of ill health.

The household forms used in 1851 were more detailed than those of 1841. For the first time, the census asked whether a person could read and write Irish as well as English, and it added a separate return, Form C, for recording sickness in the household. Fewer than one in four people on the island were recorded as Irish speakers, the first national figure of its kind.

Set against the 1841 total, the 1841 census commissioners had themselves projected that, at the ordinary rate of increase, the population by 1851 should have reached roughly 9,018,799. The actual count of 6,552,385 left a gap of close to 2.5 million people who were, in the commissioners’ own arithmetic, unaccounted for by births and deaths alone. That gap is the starting point for every subsequent attempt to separate famine mortality from emigration.

The census’s own regional tables show the loss was not spread evenly. Connacht’s population fell by 28.8 per cent between 1841 and 1851, against 22.5 per cent in Munster, 15.7 per cent in Ulster, and 15.3 per cent in Leinster. The four western counties, closest to the smallholdings and the single-crop dependence the blight destroyed, absorbed nearly double the national rate of loss.

Counting the dead: how the census measured mortality

The mortality data came from a section of the census usually referred to as Wilde’s Tables of Death, published as part of the census reports in 1856. Wilde asked surviving householders to record every death in their family over the preceding ten years, and cross-checked the results against hospital, workhouse, prison, and dispensary records where they survived.

The tables recorded 1,361,051 deaths between 1841 and 1851, with about 73 per cent of them falling in the five famine years. Fever was the single largest recorded cause, responsible for 222,000 deaths, more than 16 per cent of the total. Dysentery, diarrhoea and starvation itself made up most of the remainder.

Wilde knew the figure undercounted the true toll. Whole families died out and emigrated together, leaving no one to report their deaths, and returns from several fever hospitals, including those at Athlone, Cootehill, and Fermoy, had been lost before the census takers arrived. Later economic historians, among them Cormac Ó Gráda and Phelim Boyle, reworked Wilde’s raw returns and put excess famine mortality at roughly one million, a figure they described as an informed estimate rather than a count, since Ireland had no system of civil registration at the time to check it against. Full civil registration of births, marriages, and deaths did not begin until 1 January 1864, thirteen years after this census was taken.

Wilde compiled the tables while running his own Dublin eye and ear hospital and editing a medical journal. He married the poet Jane Francesca Elgee, who wrote under the name Speranza, in 1851, the same year as the count. Their son Oscar Wilde was born three years later, in 1854.

Mayo population loss: one of the steepest declines in the country

Connacht suffered more than any other province. Census-based reconstructions attribute 40.4 per cent of all excess deaths in Ireland, close to 437,000 people, to the province’s four counties.

County Mayo’s own figures show why. According to Mayo County Council’s history of the famine, the county’s population fell from 388,887 in 1841 to 274,830 in 1851, a loss of about 114,000 people, close to 29 per cent, in a single decade. The number of inhabited houses in the county fell over the same years from 70,542 to 49,073, a loss of more than 21,000 dwellings, a figure that reflects deaths, emigration, and the physical clearance of cabins together. Mayo’s Famine-era relief was organised through five Poor Law unions, Ballina, Ballinrobe, Castlebar, Swinford, and Westport, each with its own workhouse.

Housing as a second measure of the loss

The census did not rely on population figures alone. Enumerators also classed every dwelling into one of four grades, from first-class houses down to fourth-class cabins, defined in the 1841 census as single-room mud-walled structures with no windows worth recording. That classification, repeated in 1851, gives an independent check on the population figures.

In 1841, fourth-class cabins housed 42.46 per cent of all Irish families, according to a later statistical analysis of the housing returns published in the Journal of the Statistical and Social Inquiry Society of Ireland. By 1851 that share had fallen to 21.77 per cent, roughly half its former proportion. Some of that fall came from clearance and consolidation of smallholdings into larger farms after the Famine, and some from the simple fact that the families who had lived in the poorest cabins were disproportionately the ones who died or emigrated. Mayo’s own house count, down by more than 21,000 dwellings in the decade, sits inside that national pattern.

Consolidation had legal help. The Incumbered Estates (Ireland) Act 1849 set up a dedicated court to sell off land belonging to landlords who could no longer meet their debts after years of unpaid rents and famine relief costs. Around 3,000 estates changed hands through that court by the mid-1850s, mostly to speculators rather than the reforming landlords the Act’s supporters had hoped for, and sitting tenants had no legal protection once a sale went through. The census’s housing tables, taken in the years this process was underway, are one of the few contemporary measures of how far smallholdings had already been swept away.

Irish population decline 1841 to 1851: deaths, departures and unborn children

The 1,622,739 net fall recorded by the census is made up of three separate quantities that the census itself could not fully divide: deaths above the normal rate, people who emigrated, and children who were never born because parents died, emigrated, or postponed marriage.

Wilde’s own tables accounted for a large share of the fall through recorded and estimated deaths. The residue, once births lost to delayed marriage and reduced fertility are also taken into account, is generally attributed to emigration, though the census provided no independent count of emigrants at ports of departure. Contemporary shipping and passenger records, kept separately by port and harbour authorities, are the primary sources historians use to check that residual figure, and they do not always agree closely with the census-derived estimate.

This is the arithmetic every county figure in this article ultimately rests on: a starting population, a projected population had ordinary growth continued, an actual population counted on census night, and a set of deaths reconstructed from family memory and institutional ledgers a decade old. Emigration was never counted directly. It is what is left over once the other three numbers are set against each other, which is exactly why historians treat it as an estimate rather than a headcount.

What historians cannot know

The household forms filled in by families across Mayo, and almost everywhere else in Ireland, in March 1851 no longer exist. Most of the original returns for 1821, 1831, 1841, and 1851 were destroyed in the fire at the Public Record Office in the Four Courts in 1922, during the Irish Civil War. What survives is the commissioners’ printed statistical tables, county by county, not the forms individual households completed.

That means historians work from summary figures rather than named individuals. The mortality data was self-reported ten years after the fact by families who had lived through the events they were describing, a method that produces plausible totals but cannot be verified household by household. Families that died out entirely or emigrated as a unit left no one behind to answer the census at all, which is one reason every scholarly reconstruction of the death toll, from Wilde’s original 1856 tables to the later work of Ó Gráda and Boyle, treats its own number as a minimum rather than a final count.

Nor did the census distinguish, in any reliable way, between a family that had emigrated together and one that had died together. Both simply stopped appearing on the returns. Ireland had no system of civil registration for births, marriages, and deaths until 1864, so there was no independent record against which the census commissioners could check their own arithmetic. Every figure in this article, from the national total down to Mayo’s house count, is therefore a documented estimate built from the best surviving evidence, not a closed and final tally.

Why the record still matters

The 1851 census is the reason the scale of the Famine can be stated in figures rather than impressions. It is also, because of what it cannot show, part of why family history researchers so often reach a dead end at exactly this decade. Of the scattered individual returns that did survive the 1922 fire, none come from Mayo. Anyone researching a Mayo family in this period works entirely from the aggregate county tables covered here, not from a single surviving household form.

Anyone tracing an emigrant ancestor through the Irish emigration records that survive the same fire and the same gaps in reporting is working with the same incomplete record the census commissioners left behind. Mayo’s own losses are covered in more detail in the county’s emigration history, and the conditions inside one of its workhouses during the worst famine year are set out in a separate account of Black ‘47 in Ballina.

Sources and further reading

• Central Statistics Office (Ireland), “Census of Ireland 1851” historical statistical reports archive, including the county reports for Mayo, Louth, Carlow, and Queen’s County, digitised 2023.
• National Archives of Ireland, “Census Records 1821-1891” and related guidance on surviving pre-1901 census fragments and the 1922 Public Record Office fire.
• Mayo County Council, “The Great Famine,” local history and historical events section, mayo.ie.
• Census of Ireland for the year 1851, Part VI, General Report, Volume XXXI, British Parliamentary Papers, 1856.
• Journal of the Statistical and Social Inquiry Society of Ireland, “Housing of the People of Ireland during 1841 to 1901,” Trinity College Dublin digital repository.
• Irish Genealogy (General Register Office), “Civil Records,” on the start of full civil registration in Ireland in 1864.