The 1901 and 1911 Irish censuses include a question about the ability of respondents in a household to speak the Irish language, an Ghaeilge. The Irish language had declined dramatically over the previous 300 years as a result of English rule. The Great Famine of the 1840s was another nail in its coffin. By 1911, about one in six of the population replied they could speak Irish.
The dawn of the twentieth century, however, saw an increased interest in national and cultural identity. The Gaelic Revival was growing throughout the country, with Irish classes being run by the Gaelic League (Conradh na Gaeilge), an organisation founded in 1893 by Eoin MacNeill and Douglas Hyde. The Gaelic Athletic Association (GAA), founded in 1884 to promote Irish sports such as hurling and gaelic football, was fostering dual sports and language clubs.
Some nationalists saw an opportunity to make a political statement by completing the 1911 census returns, in particular, in Irish only. Census enumerators, who were members of the Royal Irish Constabulary and the Dublin Metropolitan Police, often crossed out the Irish and overwrote entries in English. Some of these returns were written in an old Irish script, making them difficult for computer searches. This explains why some people cannot be found on the census using the standard Search function – the Browse facility, and ingenuity, needs to be employed. Let’s take two notable examples.
Éamonn Ceannt was one of the signatories of the 1916 Proclamation. In 1901, he was returned as ‘Edmund Kent’, Fairview Avenue, Clontarf, Dublin. But in 1911, he was living in Kilmainham, Dublin, and had returned his census in Irish (http://www.census.nationalarchives.ie/reels/nai000129689/ ). ‘Oscar Traynor’, who also fought in the 1916 Rising and was later a long-serving Irish government minister, cannot be found in the 1911 census. But ‘Osgar Ua Tréinfhir’ is there, living at Ballybough Road, Dublin ( http://www.census.nationalarchives.ie/reels/nai000041523/ ).
The 1901 and 1911 Irish censuses are freely available from the National Archives of Ireland website www.census.nationalarchives.ie . Perhaps your grandparents or great grandparents responded in Irish.