Almost one third of all births were outside marriage last year, new Central Statistics Office (CSO) figures show.
The proportion of births outside marriage has increased from 22 per cent to almost 33 per cent over the last decade.
The highest percentage of births outside marriage last year was in Limerick City (53.7 per cent), while the lowest was in Cavan (21.7 per cent).
The figures are contained in the vital statistics on births, deaths and marriages for 2004, published yesterday by the CSO.
The rates were higher in urban areas, although parts of south Dublin, notably in Dún Laoghaire-Rathdown, bucked this trend with a rate of 26.5 per cent.
Sociologists say the increasing trend of births outside marriage, in recent years in Ireland, follows trends throughout northern Europe.
Some of the main factors contributing to the rise, they say, are changing social norms, attitudes towards the church and socio-economic factors.
The youngest average age for giving birth was also in Limerick (26.2 years) while the oldest was in Dún Laoghaire-Rathdown (30.1 years). There were 53 births among teenage women aged 15 or younger, one of whom already gave birth before.
There were 61,684 births registered in 2004, an increase of 167 on 2003. The 2004 total is 27 per cent higher than in 1995 when 48,530 babies were registered. This represents an annual birth rate of 15.3 per 1,000 of the population, compared to 13.5 in 1995.
While the birth rate has increased, it is not at a level which would replace the population in the long run.
A fertility rate of 2.1 is generally considered adequate to replace itself in the long run, discounting migration. The rate for Ireland last year was 1.95 - below the replacement level.
The number of infant deaths fell slightly last year with 300 deaths, a decrease of 11. The number of neo-natal deaths, or deaths of infants under four weeks, fell by 17 to 214.
These reductions are also evident in mortality rates for infants. In 2004 the infant mortality rate was 4.9 per 1,000 live births, compared to 5.4 in 1995.
The neo-natal mortality rate in 2004 was 3.5, compared to 4.6 in 1995.
In addition, there were substantial regional variations in infant mortality rates, with Limerick City's rate (12.2 per 1,000 live births) more than twice the national average (4.9).
The lowest infant mortality rate was recorded in south Tipperary (0.0) and Meath (2.2).The highest infant mortality rate, by health board area, was 7.2 per 1,000 live births in the mid-west, while the lowest was in the northeast (3.7).
Overall, the number of marriages in 2004 was 20,619, equivalent to an annual marriage rate of 5.1 per 1,000 population.
The figure is the same as last year, but is almost one third higher than in 1995 when just over 15,500 marriages were registered. The estimated overall population in April 2004 was just over four million.