Census enumerators began collecting completed census forms from hotels, hospitals and nursing homes this morning.
They will also call to households today and will continue to do so up to 23 May.
All households were required to fill out the forms, detailing the people staying in the dwelling last night, Sunday 24 April.
People are obliged by law to fill out the forms and all data collected is kept confidential.
CSO Senior Statistician Deirdre Cullen says Census forms should be completed by the public and ready for collectionhttps://t.co/d5bT353U2A— RTÉ News (@rtenews) April 25, 2016
The senior statistician with the Central Statistics Office has urged people to cooperate with the enumerators, adding that it is an offence not to do so.
Speaking on RTÉ's Morning Ireland, Deirdre Cullen said if people did not receive a Census form they should ring 1850 201 604 and arrange to have one delivered.
She said that once the forms are returned to the CSO in Dublin, staff will begin scanning and capturing the information immediately.
Ms Cullen said the CSO is aiming to have preliminary results available during the first or second week of July.
These will give an indication of net migration over the past five years and will include vital information on housing stock and, importantly, the number of vacant dwellings across the country.
She added that it will then take six months to analyse the information from the 1.8 million forms.
Scanning of the forms, at a rate of 360,000 pages a day, will be completed by Christmas and the results will be published in March 2017.