Nearly one out of every five births in the first quarter of this year was to a cohabiting couple, according to the latest figures from the Central Statistics Office (CSO).

According to its quarterly Vital Statistics report, 3,622 births were to unmarried parents with the same address, which amounted to 18.2pc of all births.

Overall, there were there were 6,768 births registered as outside marriage, which accounted for 34pc of all births, again roughly the same as for the first quarter in 2010.

The number of births outside marriage in Ireland has caught up with the EU average.

The highest percentage of births outside marriage occurred in Limerick City at 52pc, while the lowest percentage occurred in Leitrim at 21pc.

Limerick City consistently registers the highest rate of out of wedlock births in the country followed by Cork city at  48pc, Waterford city with 46pc and Wexford and Dublin City at 42pc.

There were 19,950 births registered in the first quarter of 2011, an increase of 7.6pc in the number of births registered in the corresponding quarter of 2010. This is the highest number of births registered in a quarter since the series began in 1960.

There were 3,052 marriages registered in the first quarter 2011, 329 less than the corresponding quarter of 2010 and an increase of 568 from the number of marriages registered in the first quarter of 2002.

This is roughly the same percentage as for the corresponding quarter last year.

Studies have shown that cohabiting parents are far more likely to break up than married parents.

For example, 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.