People in Northern Ireland are older and more likely to be atheist and divorced than their neighbours in the Republic.
A new census report systematically compares life in the two jurisdictions for the first time, highlighting key differences and similarities in religion, lifestyle and work.
It shows that Northern Ireland is much more densely populated than the Republic, with 134 people per square km compared with 78 down south.
However, the population of the Republic is growing much faster and reached 4.6 million in 2011 compared with 1.8 million in Northern Ireland.
The average age in the Republic is 34 – the youngest of any EU state – while in Northern Ireland the median age is 37, which is also comparatively young.
Marriage and living together are both slightly more common in the Republic while splitting up and lone parenthood are more common in Northern Ireland.
Nearly one in 10 adults in Northern Ireland is divorced or separated – which is nearly twice as many as in the Republic – and 37% of northerners are single compared with 42% of southerners.
Not surprisingly, when it comes to religion there are striking differences.
Protestants and other Christians comprise 42% of the population of Northern Ireland while Catholics account for 41%, and one in 10 people has no religion.
Around two-thirds of Northern Ireland's population were Protestant in the first half of the 20th century but this has changed hugely since the 1960s as the number of Catholics increased and Protestant numbers fell.
In the Republic, however, 84% of the population is still Catholic and just 5.9% has no religion. The proportion of Protestants has fallen by a third to 6.3% over the last century.
The report is published by the Irish Central Statistics Office and the Northern Ireland Statistics and Research Agency, bringing together census data carried out just weeks apart in 2011.
"This report offers a rare opportunity to present a detailed picture of the populations of both jurisdictions at a single point in time," CSO senior statistician Deirdre Cullen said.
People in the Republic were much more likely to consider themselves in good health, with only 1.6% reporting bad or very bad health compared with 5.6% in Northern Ireland.
Unemployment was twice as high in the south at 16% compared with 7.5% in Northern Ireland in 2011.
Retail, health/social work and manufacturing employed the most people in both jurisdictions.
The censuses also showed that 8,300 people commute regularly from the Republic to Northern Ireland for work or study, while 6,500 go the other way.