Only one in five Catholics now attends Mass on Sundays in the Diocese of Down and Connor, once a stronghold of the Church in Northern Ireland.
The figures are taken from a diocesan audit of Mass attendances recorded in autumn 2011 across the diocese's 88 parishes.
Numbers are as low as four per cent in some parishes. Overall attendance at Sunday Mass plunged to 20 per cent in less than a decade.
Diocesan spokesman Fr Edward McGee said the picture that has emerged is "consonant with that in other parts of Ireland and Europe".
He added that the figures provide a detailed picture of challenges faced by clergy.