The number of Catholics in the Republic of Ireland fell by over 130,000 between 2011 and 2016, according to the latest Census data.
The proportion of Catholics in the population fell from 84.2% to 78.3% in the same period.
Persons born outside Ireland make up 12% of the country's total Catholic population.
The number of those saying they had no religion increased by 74%, from 204,151 to 481,388.
They now make up 10% of the population, making them the second largest category in the breakdown of affiliations under the "Religion" heading in the Census.
There were 63,443 Muslims in April 2016, up from 49,204 five years previously.
More than four in ten of Ireland's Muslims (43.5%) lived in Dublin city and its suburbs.
Other religions to experience significant increases were Orthodox Christians (up by 37.5% to 62,187) and the Apostolic and Pentecostal category, which increased from just 3,152 persons in 2002 to 13,350 in 2016, an annualised growth rate of over 10.9%, according to the Central Statistics Office, which carries out the Census.
However, the Church of Ireland, with 126,414 members at last year's census, experienced a 2% decrease.
The largest concentration of members was in Cork city and county which accounted for 9.1% of the total.
The average overall age of members was 40.3 years, nearly three years above the average for the general population.
There were 24,211 Presbyterians, a slight fall on the 24,600 five years previously.
'White Irish' remains by far the largest ethnic group in the Republic, accounting for 3,854,226 - or 82.2% of those usually resident.
This was followed by persons with "Any other White background" (9.5%), non-Chinese Asian (1.7%) and 'other including mixed background' (1.5%).
The 19,447 persons with a Chinese ethnic/cultural background made up 0.4% of the usually resident population, while persons of mixed backgrounds (70,603) constituted 1.5%.
The vast majority (94.1%) of White Irish people were born in the Republic. Of the 5.9% (226,078) born elsewhere, over half (121,174) were born in England and Wales and 53,915 were born in Northern Ireland.
Over one in three of those with African ethnicity (38.6%) were born in Ireland (22,331 persons), along with 31.3% (2,126) of those with other Black backgrounds.
Among those persons with Chinese ethnicity, over half (55.7%) were born in China, while 8.3% were born in Malaysia and 6.4% in Hong Kong.
Of those with any other Asian background, 22.4% were born in India, followed by 16.1% in the Philippines and 13.7% in Pakistan.