THE typical couple under 45 live together until their first child is born -- and then swiftly get married, new research shows.
The woman is more likely to be 2.3 years younger than her partner, better educated and holding down a better job, the Economic and Social Research Institute adds, following a detailed analysis of the 2006 Census.
The fact that women tend to be better educated than their partners is a radical change from the past, the researchers say.
The research institute found there has been a four-fold increase in cohabitation in 10 years and the habit of living together now crosses all social boundaries and age groups, although it remains most popular among the poor and those who are not religious.
Over-50s also live together much more often than in the past, especially when one partner is divorced or separated, the report says.
"Older people in Ireland also took advantage of the growing acceptability of cohabitation," the researchers note.
The report reveals that most of Ireland's population growth since the 1990s can be attributed to immigrants having children, along with returning Irish people.
The birth rate among couples who have always lived in Ireland declined during the same period.
The institute did not reveal whether most of the growth came from immigrants or returning natives.
In another sign of changing social mores, people with a Leaving Certificate education now have the most children. In the past, poorly educated people tended to have the most children.
Today, the most fertile are found among those with a Leaving Cert.
Those with higher levels of education still have children later in life, however.
A third of all families in Ireland no longer fits the traditional model of a married couple both in their first marriage, the report adds. Still, three-quarters of children still live with two married parents.
A further 9pc live with a divorced or separated lone parent and 6pc live with a cohabiting couple.
The chance of living with two married parents is much higher where parents have a high level of education.
At least 2.5pc of children live in step-families and at least 1.3pc are step-children.
Ireland still has a low overall level of second relationships and remarriage.
Younger couples are more likely to bridge social boundaries, including between religious beliefs, different nationalities and ethnicities, the report says.
This may reflect a more open approach or simply reflect more opportunity, the researchers add.