Tomorrow week, Sunday, April 23rd, is census day. Each household must complete a census form in respect of everyone in the State at midnight on that day - or who arrives on Monday morning not having been enumerated elsewhere.
So visitors to the State are included along with residents.
In the last census in 2002 almost 59,000 visitors to the State were recorded as well as just under 3,900,000 residents.
Some 27,000 Irish residents were reported by relatives to be temporarily absent, but this probably underestimates the number who were away.
Our first three censuses were in 1926, 1936 and 1946.
Since then Ireland has been one of a small number of countries that have two population censuses each decade - one at the start and one in the middle of each decade.
This is because in 1956 the suspected scale of emigration led to a decision not to wait until 1961 for the next census.
A simple enumeration in 1956 that did not seek detailed data found that post-war emigration had risen to such a level that the post-1926 halt in our population decline no longer applied.
Thereafter we counted our population at intervals of five rather than 10 years in order to find out how many people were missing - or, after 1961, to determine by how much our population had risen.
Censuses were twice postponed.
The 2001 census was postponed to the following year because of foot-and-mouth disease, and I recall that when I was minister for foreign affairs 30 years ago I unsuccessfully opposed a proposal, put to the government by the Department of Finance, to cancel the 1976 census as an economy measure.
Some years later the incoming Fianna Fáil government arranged for a substitute population enumeration in 1979, and this was used for a constituency revision - the first to be undertaken by an independent Boundary Commission.
Because of the remarkable scale of the 1971-1979 population increase, the number of Dáil seats was increased from 148 to its present figure of 166, and two years later that increase helped me to win 23 extra seats for my party.
The data provided by censuses is of crucial importance for long-term policy purposes. Sample surveys, such as the Quarterly National Household Survey, provide useful short-term updates of some data, but only the censuses of the population provides a comprehensive and solid basis for long-term planning.
Questions on birthplaces have been included in censuses since 1926, but the issue of nationality was not addressed until 2002.
Because of the return of many emigrants in and since the 1990s, at the time of the last census 134,000 of those born outside Ireland were in fact of Irish nationality - as were 90 per cent of the almost 50,000 residents here who had been born in Northern Ireland.
Thus almost half of those in 2002 who had been born outside Ireland were in fact Irish by nationality.
Without this additional data on nationality the birthplace data would have given a totally exaggerated impression of the number of non-Irish people living in the State.
We now urgently need, and from this census will have available for the first time, information on the number of non-Irish people of different nationalities who are actually at work in our State, as well as on the number of dependants of such workers living here.
This is because up to recently the only data available on non-national employment here have been the numbers who have registered for work.
This, however, has allowed a misleading picture of the scale of non-national employment to develop.
I believe that this census will confirm that because 30 per cent of those registering for work do not actually start work, and because something approaching half of these migrants return to their own countries within 12 months, the scale of non-national employment is much smaller than most people imagine.
Yet until this is confirmed by a census, fears of being "swamped" by foreign workers will persist.
Concern has recently been expressed at the inclusion in this census of a new question on ethnic background. Yet this is necessary if we are to be able to identify and address problems of social integration, thus preventing a growth of racism.
There have also been concerns about another census question which asks about the number of live births to women.
This issue of fertility has not been addressed in any census during the past 25 years despite the fact that huge changes have taken place in patterns of fertility.
The form of this question is almost the same as it was in the 1981 and earlier censuses, although previously it asked only about "live" births to the existing marriage. To have changed it to include other births might have raised very difficult issues about abortion that might have made the whole census dangerously controversial.
In contrast to the census, our electoral register is known to be very defective. Many voters are missed, while others remain on the register at an address long after they have moved.
I recall 25 years ago one house on Palmerston Road, then in student bed-sits, where no less than 48 voters were then registered - the great majority of whom had long since departed.
Suggestions that the census takers should also collect electoral register data might not commend themselves to the CSO, which is understandably concerned to protect the integrity of the census which depends upon those filling in census forms remaining convinced that the information they give will be kept secret for a century to come.
What should be done about the electoral register?
I believe the Department of the Environment should finance and closely monitor a comprehensive review of the register by all local authorities to ensure that the results of our next general election will be, and will be seen to be, free from any possibility of distortion due to flaws in the register.
Under our electoral system it is common for seats to be won by tiny majorities.
In both 1992 and 2002 two Fianna Fáil seats were won by a total of less than 100 votes, and in 1987 a Fianna Fáil seat won in Dublin Central by 78 votes secured a tie in the Dáil vote on the election of the Taoiseach. Charles Haughey was then elected by the casting vote of Ceann Comhairle John O'Connell.
With the fate of governments sometimes decided by such very narrow margins, we cannot afford defective electoral registers.