Newly married couples are on average 10 years younger than couples who enter a civil partnership.
Figures on weddings in 2011 show that in a marriage the groom is on average 34.6 years old and the bride 32.5, both six months older than the previous year.
For couples in same-sex relationships the average age is 44.3, with men at 44.7 and women 43.8.
The report from the Central Statistics Office (CSO) recorded 19,855 marriages in 2011, down 3.6% on the previous year.
It was the first year of civil partnerships in Ireland after legal recognition of same-sex relationships came into force on January 1, 2011.
Records showed 536 civil partnerships in the first year of the reform, 335 involving men and 201 for women.
The east of the country was the most popular area for same-sex marriages with a third of the total in Dublin and more than 70% in Leinster.
Meanwhile, the CSO report on marriages and civil partnerships revealed that the courts granted 2 ,819 divorces in 2011, a fall of 294 or 9% on the previous year.
July was the most popular month for marriage and January the least popular, while Friday and Saturday were the most popular days with seven out of 10 marriages on these days.
Civil ceremonies accounted for 29% of marriages, the same rate as in 2010. But they were the most common form of ceremony for grooms aged 45 and over and brides aged 40 and over.
There were 2,272 marriages involving at least one divorced person in 2011.
Of the 1,072 individual partners who entered a civil partnership, 1,018 or 95% were previously single while 48 were divorcees and six were widowed.
The Iona Institute, which has a pro-marriage stance, said the figures show Ireland is seeing a collapse in marriage and now lags behind the US and the UK.
It said the marriage rate has plunged by more than 40% since 1973, from 7.4 per 1,000 people per year to 4.3 in 2011.
David Quinn of The Iona Institute said marriage in Ireland is not in good health.
"We should care about what is happening, because marriage is especially beneficial from the point of view of children and is also associated with lower levels of poverty. People suffer when marriage goes into decline," he said.
The Iona Institute said the marriage figures should be looked at alongside a rise in cohabitation, increase in divorce and one third of children being born outside of marriage.
The Gay and Lesbian Equality Network (Glen) said the report showed that the decision to introduce civil partnerships was a resounding success.
More than 2,000 people had begun these unions by June this year.
"The huge welcome for civil partnerships throughout the country shows that Ireland is ready to take the next step to provide for civil marriage for lesbian and gay couples," Glen chair Kieran Rose said.
The Constitutional Convention voted to ask the Government to legislate for same-sex marriage in April this year and the Government have until next month to decide to accept that proposal and give a timetable for a referendum.