Two million Census forms are to be filled out in homes and businesses across Ireland tonight for Census 2011.

The last Census was in 2006, and it is expected this one could provide some surprising results in terms of national well being and population size, following the economic crash.

CSO Director General Gerry O'Hanlon says it is ‘fundamentally important that we have an accurate picture of our population’.

He said: ‘We are totally dependent on people for their co-operation with the Census’ and the CSO is hoping for a 100% response.

‘The most important requirement of the census is that we get an accurate picture of our population to allow us make the right decisions now, for the future,’ he added.

Some 5,000 enumerators visited two million addresses nationwide and have established that 1.7 million homes will have at least one person present tonight when the forms must be filled in.

Including hotels, guesthouses and shipping, two million forms in total should be returned.

Those on board ships in Irish waters at midnight must also complete the forms.

Taking part is compulsory, with hefty fines for certain offences.

Minister of State Paul Keohoe has said the Census is crucial for future planning and hoped the public will ‘play their part’ in the exercise.

The Immigrant Council of Ireland also says it is vital foreign nationals living in Ireland to take part and feel included in the exercise. The forms have been printed in 21 languages including Arabic and Swahili.

The collection of forms begins tomorrow and they will be sent to the CSO offices in Swords, Co Dublin for scanning and assessment. Preliminary results will be known by July, which should reveal if population growth of the past two decades has been maintained.

A full detailed analysis will be completed by next spring.

All personal data is kept confidential for 100 years.