One in six people whose usual residence is within the State has lived abroad for over 12 months, according to the latest results from the 2002 census.
Ninety per cent of the population was born within the State and 70 per cent of people are now living in the county of their birth, statistics published by the Central Statistics Office (CSO) show.
It also shows that 76,000 people moved into the State in the 12 months prior to the census and that the highest proportion of non-Irish nationals is in 25-34 years of age group.
The population now includes some 644,400 long-term migrants who came to live here in the six years to 2002. More than half of the long-term migrants were returning Irish with 366,800 returning to the State over that period. The remainder were foreign born.
The majority of the returning Irish came from the UK.
Counties with the highest proportion of those who have lived abroad for one year or more were Mayo, at 22 per cent, Donegal with 21 per cent, Leitrim with 21 per cent and Galway with 20 per cent.
The report notes that these are counties that have been previously affected by emigration.
Of those who moved to the State in the last 12 months 44 per cent were between the ages of 20 and 29, while two out of three were single.