Figures from Catholic dioceses, compiled by the faith-based advocacy group the Iona Institute, show that between 2015 and 2025, baptisms in the Dublin archdiocese declined by 40%.
Taking Dublin out of the equation, the number of baptisms declined by 26%, island-wide, over the decade.
A summary document published by the institute has noted a fall in the number of births during the same period.
CSO figures show that births registered in the Republic fell from 65,909 in 2015 to 54,125 in 2025 - almost 18%.
Birth rates also fell 21% in Northern Ireland during the same period.
In Dublin, births have fallen more in the city and county than in the Republic as a whole between 2015 and 2025, 22% versus 18% over that period.
Census 2022 found that 65% of children aged between 0-4 years were recorded as Roman Catholic, while 16% were recorded as having no religion.
However, almost all mothers of children in this age group who identified as Catholic were still having their children baptised according to the analysis of CSO data.
This indicates that 'cultural Catholicism' remains a big factor in Irish life according to the Iona Institute.
There's a "considerable regional variation" in the baptism figures obtained.
The Dublin archdiocese saw a 40% fall over the ten years, whereas the fall in Kerry was 22.3%.
Down & Connor, which contains the city of Belfast, also recorded a big fall in baptisms (38%), as did the Meath diocese (41%), which largely covers Co Meath and Co Westmeath.
Chairman of The Iona Institute Dr Tom Finegan said the figures were not a surprise considering the rapidly falling number of births and growing secularisation.
"Given that births have fallen by almost a fifth in the last ten years, and more people are ticking the ‘No Religion’ box in the Census, we could expect nothing but a big drop in baptism figures," he said.
"However, the figures also show that ‘cultural Catholicism’ remains strong on the island. We see that as at Census 2022, 67.9% of mothers with children aged 0-4 still identified as Catholic, even though a big majority of these would not be regular Mass-goers.
He noted that according to the Vatican yearbook, over 42,000 Catholic children were baptised in Britain in 2023, despite low levels of Mass attendance there.
"The fall in baptisms must be seen not simply as the result of secularisation, but even more as a result of falling births. Ireland’s fertility rate is now just 1.5, well below the replacement level of 2.1. This should be of concern to everyone, regardless of religious belief," he concluded.