Today's census results are expected to show that the population has reached its highest level in 135 years.
Officials at the Central Statistics Office (CSO) projected earlier this year that the census would record an overall population of more than 4 million.
The last time the population reached this level was in 1871, when the census recorded 4.1 million. The last census in 2002 recorded a population of 3.9 million.
Today's census figures are also expected to show high levels of net immigration into the country, although figures relating to nationality and ethnicity are not due to be published until next year.
The census is regarded as the best measure of immigration into the country, especially since the accession of new EU states.
While personal public service (PPS) numbers are an indication of immigration into Ireland, they are not seen as a reliable way of measuring the numbers living here.
That is because PPS numbers do not show whether a person has returned to their country of origin, while many migrant workers may not have such numbers.
CSO officials expect that the results on nationality will show that the proportion of foreign nationals resident here has increased from 6 per cent in 2002 to 10 per cent, or 400,000 people.
This figure is similar to countries with a long history of immigration, such as the US and Britain.
CSO officials say they are confident about the integrity of the overall figures to be published today after recording as near to a 100 per cent participation rate as possible.
About 1.5 million forms were sent to homes around the State.
The census also included three new questions on participation in voluntary activities, family relationships and fertility.
This year's census was translated into 11 foreign languages to ensure there would be a good response from foreign nationals living here.
The languages were Arabic, Czech, Chinese, French, Latvian, Lithuanian, Polish, Portuguese, Romanian, Russian and Spanish.
As well as typical demographic matters, other questions included commuting patterns, level of education, existence of a disability and access to the internet.
The response of the population to these questions will be released on a rolling basis over the next year.
About 5,000 temporary staff were employed to carry out the census, including 4,500 enumerators.
The enumerators earned on average €2,200 for their work.
* Details of the preliminary report of the 2006 census will be available of the CSO's website (www.cso.ie) later today.