In 1841, shortly before the Great Famine, the area comprising the present Irish State had a population of over 6.5 million. The next census (1851) showed a massive decline to 5.1 million for the same area, due to deaths from starvation and disease and large-scale emigration. The outflow thus begun became a dominant feature of the population pattern over the succeeding years.

By 1961 the population of the State stood at 2.8 million, the lowest census figure on record. From 1961 onwards the pattern changed. A combination of natural increase and the commencement of inward net migration resulting from increased prosperity produced an average annual rise in population of 0.6% in the period 1981 to 1986. Between 1986 and 1991, largely as a result of the resumption of emigration, an average annual fall in population of 0.1% was recorded.

At the 1996 census, the population of the State was 3,626,087. About 60% of the people live in cities and towns of 1,000 or more inhabitants. Population densities are highest in the east and south.

The population is very young with approximately 41% under 25 years of age and 24 per cent under fifteen.