A growing number of Irish couples under 45 are choosing to live together until their first child is born, whereupon they marry new research shows.

According to the Irish Economic and Social Research Institute, which based its findings on the 2006 census, the typical cohabiting woman is likely to be 2.3 years younger than her partner, as well as better educated and holding down a better job.

The fact that a growing number of Irish women tend are better educated than their partners is a radical departure from the past, the researchers say.

The research institute found couples living together now crosses all social boundaries and age groups, although it remains most popular among the poor and those who are not religious.

The report, which was quoted in the Irish Independent, reveals that most of Ireland's population growth since the 1990's was the result of immigrants having children, along with returning Irish people. The birth rate among couples who had continuously lived in Ireland declined in the same period. The institute did not reveal whether most of the growth came from immigrants or returning natives.

The report revealed that Irish people with a high school diploma now have the most children. In the past, the less educated people tended to have the most children.

College graduates and those with higher levels of education still have children later in life, the report found. Fully one third of all families in Ireland now no longer fits the traditional model of a married couple both in their first marriage.