Methodology
What the figures measure, where they come from, and how carefully to compare them across the years.
What is measured
Each figure is the percentage of people aged 3 and over who reported, in that census, that they could speak Irish. It is self-reported ability, not a test of fluency and not a measure of how often the language is used.
Comparing across years
Census questions and definitions changed over time, so comparisons between distant years should be interpreted cautiously. The CSO notes that results such as 1926 and 2022 are not directly comparable, and a new question series began in 1996.
That is worth keeping in mind for the jump from 1991 to 1996 in the story: part of a rise that fast can come from how the question was asked or answered.
Northern Ireland
Northern Ireland recorded Irish only in the all-island censuses of 1851 to 1911 and again from 1991, so the six counties show as “no data” in between. Its 2001 and 2021 censuses appear under 2002 and 2022 to line up with the Republic; the county panel says so when that applies. Antrim and Down for 1851, 1861 and 1901 are estimated, because Belfast straddles both counties and was not split between them in those returns.
Some Northern Ireland county figures are themselves estimates. The 1991, 2001 and 2011 values were derived from ward-level returns, and NISRA’s 2021 county figures are estimated from census areas, as the North does not publish this on a county basis directly.
The history in the story
The notes beside the map and the chart give historical context. They are not findings of this project, and the census alone cannot show what caused a change. They draw on:
- On the Famine and the language: Language and Literacy: Decline of the Irish Language, and An Gorta Mór.
- On the national schools of 1831, and the debate over their part in the decline: The Irish Language in Co. Wicklow.
- On Irish as a compulsory subject from 1922: Irish made compulsory in schools (History Ireland).
- On the Northern Ireland census dropping and restoring the question: A century of change in Northern Ireland, 1926–2026 (Queen’s University Belfast).
- On the new question in 1996, and on why 1926 and 2022 are not directly comparable: the CSO’s series notes and 1926 and 2022 comparison.
The maps
The county outlines are simplified so the maps draw quickly: easy to recognise, but not a survey-accurate map. The numbers behind them are unchanged.
The census forms
The wording of each census’s question about Irish is taken from the forms themselves. The 1851 household return shown on the home page is from the National Archives of Ireland; question 13 of the 2022 household form is from the Central Statistics Office. In the “fill in the form” section, the 2022 figures for “daily at school” and “weekly or less often” are worked out from the CSO’s published totals and rounded.
Sources
- Republic of Ireland: Central Statistics Office (data.cso.ie).
- Northern Ireland: the Northern Ireland censuses, compiled by Phelim Birch.
- Boundaries: Ordnance Survey Ireland and Ordnance Survey of Northern Ireland.