Ireland's young are disappearing. Since 2008, the number of twentysomethings living in the Republic has declined by almost a quarter of a million. This amounts to the loss of almost one third of the 20 to 29-year-old age cohort in just six years. Never in this State's history have people in their twenties made up as small a share of the total population.
A shrinking of this age group on such a scale - something that usually only happens in times of war, famine or plague - can be expected to have social, political and economic consequences aplenty.
I'll return to these many consequences in a future column, but today let's focus on the reasons it has happened, not least because they are not what you might expect.
When I tweeted about these figures on their publication by the CSO last Friday, much of the reaction was on emigration. That is certainly one reason for the collapse in the number of twentysomethings in Ireland, and we'll return to it presently, but the biggest reason is one very few people seem to be aware of. That reason is a precipitous fall in past births.
As the accompanying chart shows, there was a sudden, sharp and protracted fall in the numbers of babies being born in this State from the start of the 1980s.
The downward trend continued until the mid-1990s, before going sharply into reverse and peaking in 2009 (there appears to be a strong link between how the economy is doing and the birth rate, something supported by the fall-off in births since the property crash happened).
So, about half of the recent decline in young people in Ireland (230,000 between 2008 and last year) is the consequences of fewer babies coming into the world back in the 1980s and the first half of the 1990s.
The good news from this is that the contraction in the number of twentysomethings will go into reverse as the boom-era babies start leaving their teens, a trend that is just starting to happen.
Now consider the less benign of the main culprits for the incredible shrinking twentysomething cohort: emigration.
It is not surprising that most people's reaction on hearing the news of such a change in the number of resident young people is to point to emigration, such is the prominence of that phenomenon in the national psyche.
Ireland's unique history of outward migration explains this prominence.
From my trawling of global population databases, I can find no other country anywhere on the planet which has a smaller population now than in the first half of the 19th century.
Not even in the poorer regions of peer countries has there been population shrinkage of the kind Ireland has experienced.
Scotland and Andalusia (one of Spain's poorest regions) have experienced no outright depopulation in the period since the 19th century, despite lots of emigration to the less peripheral and more prosperous parts of Britain and Iberia respectively.
Even Italy's comparatively poor islands - Sicily and Sardinia - have managed to grow their populations despite losing many of their young to the lure of the mainland and the Americas.
The strength of the Irish impulse to migrate in unusually large numbers began with the Famine. So big was the outflow thereafter that the State's population fell almost uninterruptedly up until the early 1960s, which - to repeat for emphasis - was unique among the nations of the world.
Matters have improved a great deal over the past half century, but such is strength of the tradition that emigration quickly returns when times turn tough - as was to be seen in the 1980s and in the post-2008 period.
But despite most people's instinctive reaction to attribute most of the fall in the young population to emigration, the CSO estimates show that the numbers of twentysomethings coming and going (immigration less emigration) since 2009 has been around 70,000. In other words, only around 30pc of the massive decline in that age group is accounted for by migration.
The third significant factor has been the aging of immigrants who arrived in Ireland in large numbers, particularly after 2004, as Seamus Coffey, an economist at University College Cork, quite correctly raised in a discussion on Twitter earlier in the week. Because the tens of thousands of young Poles, Lithuanians and others who arrived in the last decade have been followed in much lower numbers of the next generation of their compatriots, the migrant population has aged.
Having swelled the ranks of the twentysomethings in the last decade, immigrants are now moving into older cohorts.
If many readers are surprised at all of this, they might be even more surprised by the reason people move abroad.
The classic emigrant in the mind's eye of many is the unfortunate who has lost his job or the luckless kid who has left education and just can't find one.
Thankfully, the sort of jobless desperation that forced people onto boats in the past is much less common today. The State's statisticians find that just one in five emigrants in the 2009-14 period was unemployed. Of the other 80pc, most had jobs. The other main group was students, who appear to have pushed off as soon as they had their degrees in their hands.
Ireland has had eyebrow-raising demographics for a couple of centuries. That pattern continues today, and looks set to continue for at least a little while to come.