Almost one fifth of all births in Ireland were to cohabiting couples in the first quarter of this year, according to new figures from the Central Statistics Office (CSO).

The figures, contained in the CSO’s quarterly Vital Statistics report, show that there were 6,405 births (34 per cent of all births) registered as outside of marriage in quarter 1 2009.

Of these, 3,354 (18 per cent of all births) were to unmarried parents with the same address.

A poll commissioned by the Iona Institute in January showed that a clear majority of people continue to believe that marriage is better for society and for children than cohabitation.

The poll also found that over 90 per cent of the public support a child’s right to a mother and a father, where possible.

According to the British Millennium Cohort Study, only 10pc of married couples will have broken up by the time their child is five, compared with 25pc of cohabiting couples.

Only 35pc of British children born into a cohabiting union will live with both parents throughout their childhood, compared with 70pc born to married couples.

According to the only piece of Irish research on cohabitation, only one in four cohabiting relationships in Ireland last seven years or more. The rest end in marriage or breakup.

Couples who cohabit before marriage are more likely to divorce than those who do not cohabit first.

Overall, there were 18,787 births, 42 per cent of which were to first time mothers, 39 per cent of which were to second time mothers and 17 per cent of which were to third time mothers.

The average age of mothers was 31.2 years, ranging from 29.1 years in Limerick City to 33.0 in Dun Laoighaire Rathdown. The average age of mothers having their first child was 29.0 years.

There were 3,450 marriages registered in quarter 1 2009, an annual rate of 3.1 per 1,000 population, 0.1 above the corresponding quarter of 2008.