MIGRATION is often said to occur in "waves". The description is apt. The flow of people across borders is constant, but not steady.
It can be hard to predict or control. In stormy times, the effects can be spectacular and exciting, leaving a lasting impact.
As a small country with a turbulent economic history, Ireland has seen large waves of migration -- in and out, in good times and bad.
We are unusual in conducting a Census every five years and, despite the cost, it is good that we do. Without a regular and thorough count of who is here, it would be much harder to discern what is happening to our country and its people.
The Census form asks every person to tick a box to describe their ethnic and cultural background.
The category "white-Irish" probably comes close to capturing, though imperfectly, Ireland's long-term native population. The results of Census 2011 reveal how that group has been hit by this massive recession.
At the time of Census 2006, there were 530,000 "white-Irish" aged 15 to 24.
Many of these people would have been in first jobs, looking for a first job, or perhaps expecting to graduate in an Ireland that appeared to offer better career opportunities than ever before.
Yet five years on, Census 2011 records just 490,000 "white-Irish" aged 20 to 29. In other words, over the five-year period at least 40,000 of those young people disappeared.
The numbers equate to at least 10pc of men and 5pc women in this group emigrating. The true amount of emigration in this group is probably higher, because some would have been abroad in 2006 and returned.
Yet most of the flow was clearly in the other direction.
Doubtless many of the men in their late 20s had worked in construction, or perhaps other jobs crushed by the property collapse. But emigration was actually highest among those aged 20 to 24 in 2011, of whom at least 11pc of men and 9pc of women have gone, often directly after leaving school or college.
The picture for people of other backgrounds looks different. Between 2006 and 2011, the population increased in all other ethnic categories.
This does not mean that non-"white-Irish" were less likely to leave once the recession began to bite in 2008.
Rather, it mostly indicates how many immigrants continued to arrive in Ireland after 2006, especially from Eastern Europe.
By April 2011, those who had arrived after April 2006 still outnumbered those who left once the recession hit, resulting in population growth for the five-year period as a whole.
From one perspective this is surprising, because recent immigrants to Ireland have experienced higher levels of job loss than their native counterparts.
Yet jobs may be only one factor in the decision to stay or go.
The new Census 2011 data offers hints about what else matters. The largest population growth since 2006 occurred among children and people aged around 30 or just above, especially women. Among recent immigrants, it looks like some men who kept their jobs were joined by their families.
Most immigrants to Ireland originate from countries with considerably lower incomes and from cultures where people usually marry and have children at a younger age.
So, despite their more recent arrival in Ireland, the incentives they face can be quite different from the incentives faced by members of the long-term native population.
Irish people have become used to a high standard of living. They form partnerships and have children late by international standards.
Emigration is less unattractive to ambitious single people.
Lastly, whatever their ethnic category, the adults who left have tended to be younger than those who had previously arrived. Consequently, the peak in Ireland's age profile at around age 30, which began with a mini-baby boom in the late 1970s, has become even sharper.
We now have 32pc more 30-year-olds than we have 45-year-olds; 44pc more 30-year-olds than we have 15-year-olds. This is why the birth rate is so high.
Thus, the recent waves of migration, first to and then from Ireland, are leaving permanent tide marks on our land.
Dr Pete Lunn is a Behavioural Economist at the ESRI