Belfast occupies a unique position within Irish economy and society, not least because of its status as the lone industrial outpost of any scale on the island of Ireland. Consistent with this commanding position it attracted short-distance migrants to the city, and hence its youthful population, its low levels of emigration, and a gender ratio biased towards women. By virtue of its high-technology industries, which were geared towards exacting export markets, the city housed a substantial ‘labour aristocracy’, with up to a quarter of male workers in 1911 classified as skilled workers. To pursue the demographic characteristics of Belfast in more detail, we turn to the single most revealing set of sources on Belfast society during the Edwardian period, that is, the manuscript returns of the censuses of population for 1901 and 1911. These are generally believed to be reliable across a range of categories of information, from birthplace and religion, to literacy, occupation and marital status, and this indeed appears to be the case. However, we can be less sanguine about the ages reported in 1911, which in turn are vital to the study of issues such as fertility, mortality, age at marriage, age differences between spouses, and the age structure of the population in 1911. We find that age misreporting at the time of the 1911 census was serious for older people, particularly so when one seeks to compute measures such as the mean age at marriage of men and women, either for the population as a whole or broken down by social class and religious affiliation. The misreporting of ages seems to have been most acute in the Catholic community, and particularly amongst women. We have, therefore, corrected the age information reported in 1911 by reference to the ages recorded against the same individuals in 1901, when the distorting effect of the introduction of the Old Age Pensions’ Act of 1908 was not operative. Having corrected the age information, it is apparent that Belfast does not fit neatly an Irish template of demographic behaviour. The frequency of marriage was higher and age at marriage was appreciably lower than in Ireland generally. Broadly speaking, these variables conform to English and Scottish patterns, though the high incidence of non-marriage among women in Belfast is worth highlighting, lying as it does somewhere between Irish and British levels. Fertility among couples in Belfast was generally high, though somewhat lower than in Ireland generally. Fertility varied by age at marriage, something that might be expected. But it also seems to have varied by religious background. The econometric evidence indicatesthat Protestant couples in Belfast had smaller completed family sizes, as compared to Catholics, which in turn suggests that the great divergence between Protestant and Catholic fertility – so evident in Northern Ireland in the later twentieth century – had its origins a century earlier. Funding: Heritage Lottery Fund, Northern Ireland: grant of £122,000 towards a database of Belfast in 1901 and 1911.